Short answer- Yes, but no.
As mentioned before, fast swapping will allow you to switch between pokemon very quicky (Not so much with zard). However, you would be doing yourself a huge disservice by not using all three. Squirtle struggles killing at higher percents and sometimes can have issues with chars with longer range (even though leffen has went on record saying squirtle does pretty good v. Chrom). Zard and Ivysaur both are great at those repectively.
In addition, Pokemon Trainer's offstage game is very potent when using all three mons with the power of quick swapping. Ivysaur's Down-Air has such a huge hitbox and his Forward Air's tipper has a lot of knockback. Zard has multiple jumps, which lets him go out a littler further when connecting a fair, bair, or dair to kill.
Squirtle's recovery is sorta mid too. Ivysaur's recovery on the other hand is very good.
Tldr: Do what you want. However, you are selling yourself short if you choose to ignore Ivysaur and Charizard.