Since you're from the same region as Reflex I may have to take your word for it. I always assumed that mastering at least 2 Pokemon was critical due to the stamina system.
You need to master all three if you hope to be effective with the character.
Squirtle probably has the most hope out of the three of standing alone, but you'll still lose to the likes of
![Marth :marth: :marth:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
and the occasional
Ivysaur is, in no way, a stand alone character. her easy juggle-ability, poor recovery, and sub-par matchups make sure of that.
However, she does do a good job of covering some of squirtle's worser matchups. Namely Ike, DK, and Bowser.
Same for Charizard. He's good in some ways, but when you're countered by
![Toon Link :toonlink: :toonlink:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
ect...
You see where I'm going with this.
You
can attempt to master only two.
But if you leave out Ivy, you'll be down one of your best K.O. grabbers out of a switch. Furthermore, you'll have to switch twice to bring back charizard should he die in one of squirtle's lesser matchups.
You can try to drop Charizard, except you'll lose badly to
There are other problems, but I'll stop here.
Atempt to master only 1 or 2 at your own peril.