Couldn't help but to see this with "Pit" on it, so I can only help out a bit only because I don't see many PTs these days.
Squirtle is generally your best choice, considering how Pit has problems trying to hit smaller chars, aka, ur little water turtle.
This is especially if you can't space well with Ivy against Pit, and Charizard's a big target for Pit, not to mention Pit to have one of the best edgeguards and camping games in the game, capable of gimping Ivy and Zard quite well. Squirtle, being small, is harder to hit, along with pretty good speed, and therefore is the most difficult for Pit to deal with.
It kinda depends on the Pit and his style, but it's hard for me to really explain it. All I can say is ur best bet is to harass Pit with Squirtle first, doing as much as you can to him, then go for some quicker kills with one of the other 2. Just be careful of Pit's great gimping potential against Ivy and Zard.