They just seem to get dominated by all the fighters like xin zhao. Anyone with gapclosers, hard cc or ranged harass zone them, kill them, and make them useless. But I've also seen teams who win with carries, and even in normals a team with 2 carries (kog and sivir), who won. SO in which cases are carries good? (which carries are good, what teams they do well with, etc.)

If the game lasts a long time and goes to late game, the carry will be better.

If not, the bruiser will usually be able to kill the carry.

It's impossible to tell how long a game will last until it's lasted that long, and you also don't know how your teammates will play. You could have a really good Ashe beat a really bad Xin.

Maybe if the carry lanes with a hard CC tank, Ali, Blitz, Leona etc. That way they won't be able to really get hit by the enemy without the enemy getting knocked up, stunned, etc. We had an Ez-Leona bot lane completely dominate us once.

yes they are

Ez and teemo are the only ones that I would consider viable in the hands of a skilled player - they counter melee champs well top early game.

the best ad carries in 3v3 (in my opinion) are Tristana, (who is horribly underplayed </3) draven, and ezreal. i might say teemo too but i play him ap

You need to pick the right ones.

ADC who can do high damage to tankier targets and those who have great mobility/defence to avoid being blown up instantly.

Vayne and Corki would be the better picks. True Damage and escapes. Vayne would pip Corki Due to her knock back though.

Graves has his slow, dash and tankiness.
Ezreal has mobility and could steal objectives with his Ult.
Cait has a decent escape and traps can give you map awareness.
Tristana has an escape and a knockback, however she doesn't really reach her best spot until later on.
Twitch is an option too because he has stealth and true damage.

Trist has a very powerful MS which is invaluable against bruisers.