I mained PT before I went to Mario. Squirtle plays a lot like Mario, except that his only kill move is predictable and laggy (Usmash). Squirtle should be fairly easy to handle. His light weight makes him easy to KO, and his recovery is easily caped. I would put this match up 70-30, Mario's favor.
Ivysaur is also fairly easy. Easier to uair, but he has much better kill moves. His bullet seed can be a bit of a nuiscance if you're trying to come down from the top and Dair him. Fireballs work well here, and he also has an easily gimped recovery. However, he has better range and razor leaf can be trouble in the hands of somebody who can actually use it. I'd put this around 55-45, Mario's favor.
Charizard is easy to juggle, and easily hit with fireballs. However, his grabbing range, power, and tilts can be a problem to Mario. His weight makes him hard to K.O., but his recovery is definately easily caped. I'd say that this match-up is neutral, and that the better player should win. 50-50.