I don't know about you guys but CELO shows AT teams by giving them all the same rank when I'm in a game.
That said, my experience with 3v3s and 4v4s is you can carry your team to victory as long as you don't have to fight coordinated AT teams. Or people who are 100-1000 ranks higher in the ladder than your teammates. Otherwise, you're pretty screwed and you might as well quit when you see a teammate drop and replaced with an AI.