I'll make a few suggestions, from what I've seen. I've basically only played 3v3s with maybe 3 or 4 5v5s scattered in between. I'm only level 18 on my main, but considering I have 5 or 6 other smurfs, I've played a good 300+ 3v3s already. Because I haven't played many high level games (I duo with 2 of my level 30 friends a lot, but that's about it), I may make mistakes in my judgement just because I have yet to play against a person who is actually good at that champ. For example, I've only played against 4 or 5 Cass's, but I haven't had any trouble with them.
Cassiopeia is of course good as always, but she should probably be Tier 1, because of the lower number of squishies in 3v3s, making her less effective (it's harder to lane against an offtank as Cass)
Gangplank is Tier 3 in my opinion. As long as he isn't fed, he's really not a problem. Weak shots, random ult (as in, you can ult a runner, have them run through the whole circle, and not get a kill because they were lucky and weren't hit). If I see a GP, I just make sure he doesn't get fed. I will admit that a fed GP is incredibly OP (basically a ranged Tryndamere), so maybe he can be Tier 2.
Fizz belongs in Tier God. He's way underplayed, but has absolutely no problem in a 3v3. He can dodge all skillshots, gank extremely well, and jump many walls in TT.
Sion should be Tier 1 or God if he is AP. AP Sion is nearly invincible and nukes like crazy. But then again, I have yet to play against a team who knew to break my shield when I was going AP Sion, so yeah.
Jax should be Tier God. The only reason he's not regarded as an OP champ is because he's worse in 5s because of his lack of survivability. In 3v3, he can easily stun the whole enemy team, spam nukes with his passive + w + lich bane, and take 2 of them out within the 1.5 second stun. I have yet to lose a match as Jax where none of my teammates are extremely bad or afk. Think about it. If I'm playing Jax and I have a Lich Bane + >300 AP, this is what my attacks look like. Leap Strike > Basic (Lich Proc) > Empower (Lich Proc) > Basic (ult passive) > Ult > Basic (Lich Proc) > Basic > Basic (ult passive) and by then it's over. In numbers, that's about (I haven't memorized his damage) 250 > 370 > 700 > 500 > become invincible due to increased armor and mres > 370 > 70 > 500. And also remember that during this time, he's getting faster and faster, which means more ult passive procs.
That's about it. Hopefully you consider some of these Or correct me if I've made a mistake, I'm always open.