I refuse to go into that map when I cannot buy wards, but the enemy can pick Teemo and get them free, thus locking my team down from making any plays in secret, while his team has free 100% control of the jungle.
It doesn't even take skill. If I felt like "Wow, that Teemo outplayed me" it wouldn't be a big deal. But instead he just plonks a ward down at the entrances to the jungle, of which there are too few anyway, and suddenly all his lanes are ungankable, there is no roaming potential, and if he tries to take one of our shrines, we can't move to stop him without his team knowing.
Having a teemo against you in 5v5 is annoying, but at least you have wards too.
Having a teemo against you in 3v3 is hell. There isn't even any counter play to it. Buy oracles? 250 gold to counter something he gets free, it's temporary, and if he puts it in any of the tiny bushes, your oracles does nothing, and even if you see the mushroom, he STILL knows you're there, because you have to destroy it.
He doesn't even have the drawbacks he does in 5v5 either. In SR all he's good at doing is winning lane and then split pushing, he's a lot less useful in teamfights, he blows up too easily. Not a problem here, its only 3v3, the teamfights are more like lane skirmishes anyway. Not that he can't still split push, if you try to come stop him you'll hit 3 shrooms on the way and he'll be gone.
tl;dr Delete Teemo, and put a draft mode on 3v3. Teemo's not the only champion that warrants a ban.