I’m not sure if this is where I should ask this, but it’s worth a shot. 

First question: why are there only three attunement slots for each character? Is it to limit the amount of certain magic items one can use, or some other reason?

Second question: is it possible to add slots, specifically in the case of homebrew?

Third question: if not (to the second question), is there any way to change this/bypass it, or would the only way be to have items not require attunement to them?

Attunement slots are for balance; items that require attunement tend to be more powerful than those without. Attunement acts as a 'limiter', both in terms of how many you can have active at once, and also in terms of passing items between party members.

As for extra attunement slots, you can add a modifier to a feat that sets attunement slots to a specific number, between 1 and 6

1. It's to limit the amount you can attune to, so you can't have all the things such as the items that make your abilities 19 permenantly.

2. It is possible to add slots. In modifiers, select Set, then select attunement slots

Thanks so much! Your answers are very helpful!

detailed instructions in this post: https://www.dndbeyond.com/forums/d-d-beyond-general/general-discussion/64810-question-regarding-attunment-slots

edit: You set the number of slots, not the number of extra slots!

I wasn't really sold on the mechanic until I played a high level campaign with a lot of magic items. Attunement does a good job of limiting power and forcing you to make tough decisions about what to use.

Is there a way to have more than 6 attunement slots, my artificer has a homebrew feat that gives an extra attunemt slot, I don't thin the campaign will get to level 18 but f ut does I should then have 7.

Probably not, but you can always homebrew an item to not require attunement and just count it as your 7th slot

but what is the lore reason for attument slots?

You make a special connection with items that require attunement. Trying to attune to more than 3 makes your current items jealous and they refuse to accept a newcomer.

...Beats me, probably something to do with the goddess of Magic; it'd be the same reason why some spells require concentration. Flavor it however you like.

In any case, here's what the SRD has to say about attunement: https://www.dndbeyond.com/sources/basic-rules/magic-items#Attunement

Basically as Naruhoodie says.  The mechanical reasoning is solid (balance), but the lore reasoning is arbitrary.