True damage champs.

I find udyr particular toxic to 3v3, since he builds full health and still takes everyone out. He's like an EXTREMELY fast jax with a great stun and a great shield. I suppose with the lack of an "ultimate" seeming ult, he's not as bad.. but for some reason, I can destroy almost anyone except him.

Singed:
The small map allows him to essentially roam wherever he wishes, and most opponent types cannot touch him due to the foolishness of following a smart Singed. He can EASILY farm because his teammates will always help if he can't somehow lane, and he can jungle well once his lane is pushed too. Once he builds RoA, Frozen Heart, and Boots of Speed, he's basically unstoppable.

There's a lot of underrated champs on TT:

1: Teemo
2: Mordekaiser
3: Nasus
4: Varus
5: Yorick
6: Cassiopeia
7: Annie
8: Jayce
9: Maokai
10: Nidalee

Teemo is insanely powerful at the moment, just because having a good deal of ap will allow him to do great damage at choke points, delaying fights or taking advantage of positioning. It is very hard to deter this if the enemy team is smart enough to exploit it. Because at most team fights will be 3 on 3 teemo can disable an enemy carry for a good amount with his blinding dart while dealing good damage. Teemo is essentially a rewarding pick for turtle comps. I think maybe reducing the cd on the items that reveal stealth may help.

Nidalee is not overpowered herself, but for the same reason as Teemo bushwhack is way too strong. Because blackfire torch is so cheap and easy to build Nid can just place traps in choke points and wear down enemies while getting an advantage the enemy cannot possibly have. Also the massive armor and magic resist reduction does not exactly help either.

Wukong, Xin, and Pantheon are also very good, if not overpowered on the map due to having gap closers with powerful all ins.

I consider Cho the best off-tank on the map but he's also very strong in summoner's rift as well.

EDIT:

I don't see him too much, but I think Nocturne is a very good jungler in 3s. Ganks become godly when he gets his ultimate and he can split push when his ultimate is level 3

Just to reiterate a previous point of mine:

Riven. Riven. Riven, some more.

I main Mundo.

Very fast jungle clear, super beefy if built right, buffs to Spirit Visage make it actually viable against mostly magic compositions. Clears deal a lot of damage to the spider, and make it super easy to stick to people and close gaps, Ultimate sustain and speed buff in conjunction with speed shrine and cleavers make it very hard to escape from.

Singed for similar reasons, basically any "super tank" with hardcore pushing abilities, gap closers/slows. Cho'gath as well, mostly because in a 3v3 theres a lot less damage potential then a 5v5 and he can get really tanky and almost become unkillable.

Teemo. because LOL No wards.

I would however like to see Bulwark available on TT, I find currently theres no reason to build an Aegis, but a bulwark with the extra 200 health and Magic resist might be worthwhile.

I also think that you need to seperate Deathcap and Zhonyas again. Its super easy to go like Deathcap Cho or Teemo and wreck just because you get the huge AP and the Zhonyas active, as well as it making you pretty tanky. Playing Diana on TT with the old Blackfire Torch and Wooglets (That gave +movespeed) pre-nerfs was hilariously overpowered.

I found bruisers with gapclosers good on TT.
Akali, Xin Zhao, and Diana are amazing picks.

He can just keep on healing

Heimerdinger!!!!!!!!!

can attack with turrets, stuns's with granades and misiles give some fear xD