Those that are strongest on TT are those that are strong solo lane/jungler on 5v5. I feel like those that are king of jungle/solo top in SR are more easily countered than they are in TT (Singed is hard to pick a counter to as he snowballs into a juggernaut) but still strong. Mundo is an especially strong jungler/solo top if played right in SR.

I don't feel like any champs gain noticeable power in TT vs SR, but I think that wall jumping champs are quite a bit more versatile in TT than in SR since there are so many walls and that blink makes a big difference (Shaco)

LeBlanc. In 5's, going in that close for nuking is suicide. But on TT where jungle and lanes are compacted, she bounces over walls and into bushes like nobody's business.

Mundo can be just as OP in 5v5s, there are just more viable champs to solo top. (He can also be used to jungle, but that needs a bit of a different start)

ap yi. there is alot less CC to interrupt him.

gotta play him right tho.

its nowhere near as easy as ad yi

In my, albeit slight, experiences poppy seems to do better in 3v3. This is likely do to the fact that it is smaller, and more confined which leads to easier charge setup. I have had no good experiences with her in 5v5 though. Still, could just be me. (All games taken from normal queues, at about level 15.)