So I'm a monk, and one of my party members changed his character from lore bard to arcane trickster rogue. I love the dex roles, like exploration and scouting, but now he does that better, as well with his mage hand. Also the wizard can turn her spellbook into a snake and see through it's eyes and hear through it's ears, so it also covers another part of spying and exploration. I'm still the main character for wis, but it's not doing much for me as dex in term of fun and the concept of the character, and the casters can cover up my wis with their spells. Inside combat I am still very valuable, but outside I don't do much. What should I do? Change character?

Quote from Short_term_user >> 

 Your monk is still a lethal fighter. I don't see the problem. Was your enjoyment based on your char being the best at certain functions in the game? If so, then sure, you change your char as well. How many are in the party?

 We are a scribes wizard, arcane trickster rogue and me, the mercy monk. In combat, I am a beast, outside, I don't do much. The enjoyment is both inside and outside of combat. And I loved the dex roles (I am a martial artist and a gymnast in real life), so I based the character on things I like. If I play a different character with a different concept, it won't be less fun, but I want my current character to work well.

Changing characters to something that is better at the dex role than the AT rogue might enter you, however unintentionally, into a sort of arms race with the rogue’s player. By that I mean what’s to stop the rogue from making a character even better at dex stuff in response which would then motivate you to top that character and so on? Since you both seem to enjoy the dex role in out of combat situations, I suggest that you and the rogue ask the wizard to use the snake extremely sparingly to allow you two to explore, sneak and scout by either switching off turns or doing it together from now on.  

Are you still having fun? That’s really the most important. 
Also, there’s a lot more to put of combat than the specifics you give. 
I get it feels like someone has taken over your thing, and that’s kind of weird. But instead of giving up, you could seek out a new place for yourself. 

What's your character like? Does your party do a lot of roleplaying? If so, you can make sure to set your character apart with their personality. Even without though, if there's only 3 people in the party then you should be able to have some out of combat utility that the others can't provide. 

 Talk with your party members out of character and make sure they're aware that you'd still like the opportunity to contribute to DEX stuff. The rogue doesn't have to do all of it.

Have you considered a multi-class option? Is there an area in game play that the group is lacking or could you more support? From a role play standpoint, MCing can be a fun addition to a party. For example, is the a ranger subclass based on your campaign that will allow you to continue to use your Dex roles because you can add something else to the play. Or find a new role for the party and use your ingenuity to leverage your other skills. 

In short...grow your character out of "type" but honoring what you want the character to be. Treat this challenge as an opportunity. 

if you ask me your problem doesn't lie in the character you are playing, but in the group composition being not what you want it to be.
as a DM, this kind of attitude i try to quelch right away.
D&D isn't about group composition, its about role playing.
those who want group composition at all cost, are those who comes from a video game background and often just want the group to be helpfull and filled with helpful material. D&D doesn't work like that. if the DM wants to kill the wizard, he will kill the wizard even if there is 2 tanks pulling aggro. i once played a wizard with next to no intellect and next to no charisma and yet i was a beast at playing charisma based stuff back in 2E era... again this game is not like a video game where everything must fall in place. its first and foremost a role playing game where you play a charcater, a living being. you are the living being reguardless of your stats. ask yourself... do i let them do the job... or do i jump in and try it myself ? the later is what you seek. doesn't matter if you are 3 people doing the same shit... the DM wont let everyone roll everytime until sucess... so just go in and roll. do what you are good at.

also, dexterity is the best stat in the entire game, everyone will be good at it.
so don't expect the players to not take it. every players takes it !

so in the end, you play what you want as long as it gives you fun.
if you lost your fun, talk to your DM and group about it. explain to the others what you liked what why you are not happy anymore.
but remember, you are not the only one who dislikes something... the proof is that they changed character as well, so something is badly amist and i am getting that it might be the DM fault. so talk to the players and DM about it, tell them what you like and dont like and ask them why they changed characters to begin with. the entire group might be surprised by the answers and the DM might even get the impression that his campaign is not as good as he think it is.

exemple...
i entered a game with my favorite bugbear fist wizard... he's chaotic good and it was a great game... but... 3 sessions later i realised, they were all evil characters, my character would never fit with them. so i played it bogard and didn't tell anyone... but i talked to the DM about leaving the group and coming back with a minotaur barbarian that would be evil. the DM was happy to oblige. so when the time came, my bgbear left during the night as he was watching and everyone was asleep... i stole 1 of the players spellbook because thats the only reason my wizard was there for. and the other players turned around and said... JERK !!! then laughed. i had warned them 2 times in game, as my character was against their actions. then a fewe minutes later the group meets a minotaur, they immediately  step aside. he starts talking and ask them what they are there for. they answered and then he said... follow me... and he led them to his master. a witch who they'd be working for. aftera few discussion, the witch sends them on a mission and ask the minotaur to follow them, and to the players surprised.... this time i'm the one speaking in my minotaur deep voice... "it will be my honor to bring blood to my mistress !" the others were stunned ! they so liked it and now i had a character that fitted to the rest of the group.

talking to your DM and the other party members, is the way to go... will always be.
but tell yourself party composition is bullshit in D&D.
don't be locked to a party composition. play what you want to play, not what the party needs !

I advise you talk to the DM if you feel like your abilities are overshone - ask the DM if there's anything they can throw at you that requires you and the wizard to do things at the same time. That should get the gears turning in their heads, and make the party work together, rather than like a bag of golf clubs!

Or maybe see about all three of you working together. You can take the skill expert feat and get expertise in stealth so you and the rogue can scout ahead together. Maybe the wizard can send the familiar with you see they can keep tabs on the two of you and come to your aid if you are seen or if they found the enemy and you want to surprise them. It’s doesn’t have to be solo roles in each situation. 

You can run up walls at level 9 (not sure what level you are at now, but at least level 3) so you can get into areas easier than the others. 

Or maybe another feat could be useful to add to the depth of your character to fill a role the others don’t. I don’t know your scores but unless you dumped CHA maybe skill expert to boost CHA skills to help out in that area. 

With the limited info we have it’s hard to give too detailed of advice. But like others have said, if you are still having fun that’s the important part. If you are not then talk to your DM and group and see if there is something you all as a group can work out.

Good Luck

Quote from Skfma >>

So I'm a monk, and one of my party members changed his character from lore bard to arcane trickster rogue. I love the dex roles, like exploration and scouting, but now he does that better, as well with his mage hand. Also the wizard can turn her spellbook into a snake and see through it's eyes and hear through it's ears, so it also covers another part of spying and exploration. I'm still the main character for wis, but it's not doing much for me as dex in term of fun and the concept of the character, and the casters can cover up my wis with their spells. Inside combat I am still very valuable, but outside I don't do much. What should I do? Change character?

 If I were DMing for your group, I would try to create scenarios where both you and the rogue need to explore/scout together. Maybe two levers on opposite sides of the courtyard need to be pulled, that kind of thing.

But in the larger scope of things, 5e has a pretty major issue with martials contributing outside of combat as you increase in levels. Your wizard will only get better and better at scouting. Spells like Clairvoyance and Scrying can make stealth fairly obsolete. Spellcasters in general get a huge toolkit they can apply to noncombat or pre-combat situations that monks, fighters, barbarians, and rogues just don't have. You have skills, but casters have them too and they get spells that do what althetics/acrobatics/stealth/etc do, but better.

This doesn't mean you can't enjoy the game. But if it really bothers you to be relegated to non-plot-related roleplaying while the cleric is asking their god for guidance and the bard is turning away the bandit party with Mass Suggestion, then you should probably stick to spellcasting roles, especially in games that go into the higher levels.

A core problem with noncombat roles is that they often wind up with "You have one character who is the best at it, and other people are only involved when the best character is unavailable for some reason". Thus, in your example, either the rogue is the best in that role (and you don't have fun) or you're the best in that role (and the rogue doesn't have fun).

The usual solution is niche protection. The prior composition did that: a bard's niche tends to be social, which doesn't particular overlap with a monk or wizard. The switch from bard to rogue resulted in collision, and also left the social role empty.

A key question would be why the player switched. At a guess, the DM wasn't making much use of social interactions and thus the player felt left out. The fact that you find your wisdom irrelevant argues that the DM isn't doing a good job diversifying the challenges, as IME perception is used an enormous amount (other wisdom skills much less; I tend to shift tracking over to survival).