So for the next campaign that my husband is running, I had the idea for a cleric that has been cursed by Hekate. In essence this means that I have created 3 characters; life domain, trickery domain, and death domain, names Sammy, Samantha, and Samuel respectively. All 3 will be refered to as "Sam" every time they take a rest, or are knocked unconscious i will roll a d6 to determine which one is now "in control" 1-2=Sammy, 3-4=Samantha, and 5-6=Samuel, I want to see how long it takes the other players to figure it out, it's gonna be great. 

QUESTION: Should each character be aware of each other's existence? and should the other two be aware of what is happening while the third is in control? 

Both have pros and cons and very interesting RP opportunities. Sammy could wake terrified in bed next to a Lich after Samantha gets a nat 20 to seduce the BBEG, Samuel could get KO'd in battle then not be played for a month at which point he wakes up in the middle of a forest ready to fight, only to be super confused when everyone is just asleep around a fire. One could be super close friends with the same party member that another absolutely despises. It could also make some things a little difficult with being half way through an investigation, then waking up as someone else and having no idea what's going on. 

There's not really a bad way to play it, I just can't decide, so if anyone has any ideas for a reason it should be one over the other let me know.

Caveat: I have no expertise in regards to this topic, but...

"should each character be aware of each others existence?"–– I would say they more or less have to given that you're in a group environment where the multiple psyches "can't be hidden" from everyone else. Now it may be an RP thing where each personality discovers the existence of the others but I don't think there's any real way to keep the knowledge away from the group or the separate personalities.

"Should the other two be aware of what is happening while the third is in control?"–– for this part I don't think real-time knowledge would be appropriate, Else I would expect power struggles during major and random actions and decisions. At the same time I think a "memory" knowledge would be a real thing. So that each Sam is more or less up-to-date on what's going on (with or without moments of forced recollection) because as there may be three personalities there is still only a single brain so that each person would have access to the memories created by the others.

Maybe you incorporate some kind of physical tick representing personality change and then a verbal reaction to what a previous Sam has done.

 Hello, i know this is a bit late. I came here to ask a different question but since i myself do have DID (nos) previously known as MPD & have contemplated for a while incorporating an aspect of it myself into a future character. perhaps i can share just a few things with you & you can decide for yourself how you think it best be imployed for your own character idea ^_^

i'd like to say, that it manifests in more than a handful of different ways for each person (no not personalities, will get to it soon promise)

for myself, i was young when the doctors took notice of my referencing of "self" in plural.
as.. they, them, he, she, us, we.
& at that point in time, it was apparently standard medical practice to perform experimental medical techniques in order to,
well i'll just be bluntand say that they would not let me leave, unless i "made them believe" that i was singular. and that i would not refer to myself in plural or third person.

i do not do much research into how it manifests for others, although i have. its just not something i often discuss with someone, unless i've known them for awhile, or just want to test the relationship before i invest a great deal of time into it.

but, i found recently there is an ongoing debate, wither this would be "labeled" (not a fan of em haha) as covert DID, or simply ableism.

i do apologize for the tone this started with ^_^ but i want to provide context, and let you know i pass no negative judgements & only like to offer you some little known insight that may help you craft an interesting and fun character for your game.

so, i decided to be straight forward and up front about how it works. my standard mode, would not have said these things. as dissociation, while not exclusively a defense mechanism itself, is a manifestation of the mind adapting or attempting to cope with a situation in which it.... (can fill in the blank here) feels unable to handle, and CAN pass control over to a personality (or create one on the spot) which can handle the situation better than the previous host of the body/vessel. 

but the "switching" itself is not exclusive to stress, it could be for hobbies. or like in your example. perhaps someone has an accident/trauma, and that personality does not come back for any pre determinable amount of time. i honestly couldnt tell you all the ways a personality swap might occur, as for myself. i often regularly cycle through so many within any given moment at once, that many of us think together and must synchronize to some extent. otherwise we would display visable glitches. although for myself, many would be undetectable unless you were looking, or perhaps a trained professional that knows more about what to look for than i know how to keep suppressed. never the less. even if i am not putting in effort to allow myself to openly state these things, (where i get anxiety it could just be used against me all over again) i still do have glitches, albeit i am not nearly as comfortable as the more overt DID persons, whom have their own range and variety of manifestations. with a good share of which ill likely never fully relate to.

but indeed, there are some more over types of DID as well.

personalities, have different structures, rules and modes of operations. and you would say, it is different per person.
but many of us, would say, our systems are different from one anothers. (or eachothers, whatever u get my meaning.)

so i would say that you can get creative as heck with whatever rules seem interesting and fun for your campaign  ^_^

most of my personalities... which i generally refer to, as states. alternate states of mind. or of me/self. communicate with the others.
so we share memories. but, part of my condition does effect my memory and how readily i can access it.
i could spend years learning a language, and then poof. one day its all gone. (programming languages. multiple times forgetting and relearning)
or it could be something more immediate, like staying up all week researching chemistry and working on a specific formula & upon waking sleeping and reawakening, have lost a majority of the information or memory in regards to that research you spent such dedication in pursuit of finishing.
& yes, the staying awake for 24hrs, 2 or 3 days. etc, can grow with anxiety of wanting to finish something without risk of partial memory loss.

and its really difficult to say, if its short or long term memory that is more likely to be effected, as i once had such a strong long term memory that people called me to ask "who was in that movie", or "what was the one actors name" instead of just searching imdb. but due to a loss of close relative it exacerbated a personal problem  in which i kept mostly to myself up to this particular point in time. the condition, and a big chunk of my past is, extremely difficult to access. no matter how hard i might try or had tried. which is perhaps not uncommon for those with DID, but was one of very few instances, where a significant portion of memory is noticed or apparent to be missing. and it cause unease ( cause most stuff anyone forgets, we all assume wasnt too important anyway. albeit not always the truth of the matter) but is more of a contrast i'd say with just losing a few minutes. hours or a day here & there.. which would be more common for myself. & its also not everyday, its pretty much random. stress just increases the likely hood of a glitch, and that depends upon the stressor, as i don't buckle under the weight of just any and every pressure. (is often a stimulus or "trigger" that is particularly traumatic to the host, or their other dominant guardian personalities or even replacement hosts.) (as the switching itself is to find a suitable host that IS actually up for the task, where some others are not)

wehw. sorry it was longer than i aimed. but figured id offer a bit more insight than just saying
yes you can, its pretty open as its different for each person that would consider themselves a plural.

my apology i can't say much about the dice rolling, (being a newbie to trpg) but you could put them at random. like rolling for a roll.
(sanity may be odd word, but something to see IF its effecting the originalhost/person,
before rolling for an effect, or which personality gets effected maybe)
or perhaps could consider modifying the "sanity check" type of rolls from the Call of Cthulhu series.

hope this helps you or perhaps someone else who's considered roleplaying with multiple personalities & that you have a ton of fun & have a fair amount of factual information to help ya consider during your character building  ^_^

*disclaimer* i do not speak on behalf of all people who happen to have dissociative features and or disorders.
i kept this primarily to describing my own system in a way i felt comfortable with at the time of writing
with the goal of helping others find enjoyment, without stereotypes or media inflated dramatization.
will leave the dramatization up to those RP and having a good time

cheers!

*final edit*
oh and yes, it most definitely can be used as a buff/quirk like you described ^_^
where each personality has its own "specialties" which can be similar or vastly different in contrast to the dominant personality or personalities.

o& there is nothing wrong with naming them, i seemingly do not, because of that strong discouragement i mentioned that taught me how displeased humans might treat me for admitting to naming them against their suggestions. thus its just something i keep to myself and uncomfortable sharing with others. but for RP idk, still couldnt decide myself if i'd ask the other players to regard or treat my character like 3 (or more) separate characters. as i am not ashamed or offended if others would prefer to refer to me in the singular pronouns. i realized long ago that language particularly ours, is difficult to reflect concepts such as a plural entity which resides in a singular vessel. but that's just my take on it -wink- so i'd say its totally up to you, try what feels comfortable & perhaps discuss the outliers left over with the group for their opinion on what might be fun, or that you'd simply like to 'try it a little different today' or whatnot.

I have never been diagnosed with anything as I can't afford to go to the doctor. As I cannot diagnose myself for anything, I have been working on a character that has an interpretation of DID and I use it sort of as a character study and attempt at realizing my other symptoms within a D&D character that I can't quite place into specific diagnosis without professional assistance. It is also a Mystic, the Psionics class from Unearthed Arcana, so tying it all into telepathy and psychokinesis is cool as hell.

In my interpretation, I use your methods for when the character goes to sleep and falls unconscious but I also use other triggers specific to each identity very much like VERTEGREZ_NOZ mentions. In fact, their entire post reflects a lot of my interpretation in a general sense, and it is the details where we may differ.

Another aspect of the character is that each identity actually has different class abilities, although they are all still the base Mystic class. I'm still working on the character and they don't have a name yet. Choosing the Mystic class was also another very useful decision in that I am focusing more on trying to express multiple and varied mental states which I suffer, and although I cannot diagnose the conditions, I can express the symptoms.

Thing is, the symptoms themselves when translating to a D&D character are a bit difficult to portray consistently. The Mystic class and its various archetypes allow me to attribute different symptoms more so to one specific identity over another, helping to make each identity more unique. Because each identity also has different class features and I am also planning to restrict most or all of their psionic powers to their own orders, this also helps to make each identity more unique.

Much like VERTEGREZ_NOZ, dissociative identity disorder manifests itself differently from one person to the next. For my character, the identities have a general knowledge of what is going on while one identity is dominant, but there can be details missing even in situations where they switch mid-conversation. This is both an interpretation from what I have seen some YouTubers go through that have DID as well as a convenience in terms of a role-playing game.

I don't want my character to have to constantly ask what happened every new day or even every five minutes if the identities switch that often. Missing certain details depending on identity and circumstance fits with my own lived experiences as well as research into what some with DID have expressed, and can be a fun thing to roleplay.

I've also been working on a mechanic for them to "switch" mid-battle, although this must be done carefully, and am debating at what cost. There are videos online of people switching identities on camera and they literally take a few seconds to orient themselves and associate with their surroundings, so I am leaning towards taking an action to switch, and you cannot take any other actions this round or are stunned until the beginning of your next turn.

This is primarily for the sake of game balance while also reflecting what I have seen and experienced. If I can just switch at any moment to different class abilities then my character becomes exceedingly versatile if there is no cost associated. Also debating that switching itself may cost psi points, and it is essential that all of the identities share their pool of psi points for the day.

tl;dr

To answer your questions, yes and yes.

Consider other triggers for each identity that could cause them to switch while awake. If each of your identities all has the exact same class features, then there is no need to consider a penalty for game balance while in combat, but such triggers could possibly exist in relation to combat as well for you.

Heylo ^_^ Thanks for sharing, i quite like some of your suggested ideas myself!
did have a suggestion that may or may not be a good fit, but when you were mentioning "switching" mid combat
it had me thinking of an idea i had, as well how it could be used by the OP if they wanted, as we all maybe have a similar idea of how to maybe implement it as a game mechanic.
so i really like the idea of it requiring an action point (to switch) but i have a bit of suggestion for "switching" or how we might go about a well designed penalty system.
i'd actually still been thinking of something like what OP stated, with lets say a D6 where 1-2 is persona A, 3-4 is B, 5-6 is C for example. if covering 3 (which makes sense, can understand 100+ not being ideal for game play)
where it diverges a bit, is atm im thinking of borrowing from Call of Cthulhu trpg 's "Sanity check" system, where it could either be
A) stripped down, for chance of switching failure roll. which could be tied to a character stat, or established between DM & player negotiations.
or B) taken up a level beyond stun/lose turn (if desired) & i was thinking that on the extreme end of a "fail roll" (checking for a successful switch) could be a deck of temporary roster personalities. where the player's character could get a 'new' personality when a roll fails to successfully switch to a dominant personality.

basically, i feel it makes a lot of sense to spend AP & i'm not trying to over complicate things, was still just playing with the idea myself to see what might be fun and not too difficult to implement. but thinking about it, would also be cool if on a high success roll for successful switch that the AP point is recovered, or gets returned (+1) on next turn. trying to think of something to somewhat emulate a "seamless switch"

which not only varies person to person, but can also vary persona to persona.
which can also be for better or worse, depending upon factors. like triggers, environment stress, recent/past events.
to me sounds like an opportunity for DM & player to discuss the system, so however its rolled, the DM can make adjustments or apply small modifier to add depth or keep the system from leaning into the unfair territory somehow, such as potentially have extremely low success switching to a (main/desired) persona, some potential extreme AP/turn loss or the inverse, if DM feels there are an extreme of successful 'switches' even among a multitude of specific stressors that should in theory lessen chances for success.

these were just some ideas, ideas i can't see being codified. but i wanted to say that what y'all have posted here so far sounds very fun & healthy, and that how you've presented them i think most DM or players would be open to giving it a shot ^_^ just ya know, discuss it before hand & see what y'all agree upon (also try to be flexible lol may change w char arc progress.)
Hope each of ya has a great time & i look forward to hearing more cool ideas here hopefully in the future!
(i also didn't mean to restate some things, but that i really like the idea of using AP loss/no loss to help balance the extremes) A+ stuff here