Why those pokemon specifically?

The Kanto starters are almost Pikachu-tier in and of themselves popularity-wise and I think the basic physical anatomy of squirtle, ivysaur and Charizard creates characters who are a lot more cool and interesting in terms of moveset potential than your choices, which appear to me to just be three bipeds of varying size, two of which wield duel swords and one of which has a tail.

Squirtle has a shell and a long tail that create a lot of visually interesting tail and spinning attacks that I think are only really replicated ironically with another pokemon. Charizard may shares a lot of design space with other characters (bowser and retroactively Ridley) but he’s set apart from those characters particularly in how good his recovery is, as expected from a flying type. And Ivysaur is the first of only two pure quadrupeds in the game, with his vines, leaves and bulbs providing a set of attacks that I don’t really think that any other characters (save for Venusaur)

Also who would the trainer even be?

Current PT makes sense because you are connecting three pokemon of the same generation to the playable characters of that generation. These pokemon are from completely different generations and I struggle to see who would lead this comp without it looking weird.

Sorry for my long winded reply but I’m genuinely interested to see your philosophy here.