My idea:
Squirtle: The Weak and Fast one
Charizard: The Strong and slow one
Ivysaur: Both quick (he has four legs) AND strong(he is almost heavy like Charizard)!
PROS:
-You can change character!
-When you change pokemon at first, they will have 0 stamina!
-When you are in mid-air, you can change pokemon and get a faster recovery!
-Squirtle is the fastest, Charizard is the strongest.
-Ivysaur is the most balanced, making him an average character(as of what I see)
-Also, by assuming, I think you can miss some long ranged attacks while changing pokemon.(because you go back into a pokeball for a short time)
CONS:
-When you change pokemon, your pokemon wont have a health of 100%
-The pokemon have stamina (probably of 100%), so they will get KO'd easily when their stamina reaches 0%.
-Chances are, when you change pokemon in mid air, you will fall while changing.
-Squirtle is probably the weakest, Charizard is the slowest
-You might not change into the right pokemon when you want, so that might make you an easy target, sadly.
I hope my CONS and PROS were good...