I am thinking that Squirtle might be on of the best out of the trio for getting kills. His water attacks could provide high KO power, although low attack power. I can see Squirtle being nimble with average speed, light weight, low percentage attacks, but nice KO moves and fairly useful specials.
Charizard might have the most damage inflicting attacks. But I see him as very laggy and slow. His main strengths will be survival. Heavy weight with gliding. Mostly used as the last leg of the team.
Ivysaur I see as the average guy with range. Pretty decent all around and a good buffer between Squirtle and Charizard.
So Squirtle might gain a cheap KO (but give the opponent a way to gain a fast KO due to limited weight), then Ivy will come along for a fair fight, and then Charizard will try to go for a kill or put on damage before he is knocked off.