I will try to make my points more on the individual and not on the potential as a team.
Regarding Squirtle, I don't think he absolutely loses against Ivysaur and Snake. Ivysaur struggles mostly in disadvantage so Squirtle can potentially choke him out. I would say that Ivy has the advantage, but Squirtle can hold his own. Regarding Snake, take a look at my earlier message regarding the Snake matchup. He can combo and juggle snake forever, and watergun is good for messing up Snake's grenade traps. This is why it is hard to rank each individually, because Snake can kill Squirtle mega early and as soon as you get to that point you're gonna want to change to Ivysaur, but that's just part of the character... fact of the matter is, Squirtle can do a lot against Snake and he is a very important part to winning the matchup. I also think Squirtle does really well against Captain Falcon since he struggles against small, combo heavy characters, he can give Donkey Kong a hard time for the same reasons, same thing with Ike plus he can mess with his recovery with watergun, he does well against Roy and Chrom because they are fastfallers and he can get huge damage off them with combos, plus he can mess with Chrom's recovery even more than Ike's. I also feel like Squirtle gives Link a hard time due to Link's slow speed plus projectile heavy gameplay and fast fall speed. I don't play against many Pikachu's, but Squirtle is the go-to character against Pichu because Squirtle can match Pichu in speed, and while the matchup is just even, most including myself believe that Ivysaur loses to Pichu (doable matchup but favorable for Pichu, Ivy relies exclusively on razor leaf and bair and waiting for Pichu to mess up) and Zard gets obliterated by Pichu (like he literally cannot do anything), so it's good to just stick with the even matchup between Squirtle and Pichu. I'm guessing that Squirtle does about the same against Pikachu but Ivy and Zard both do better in that matchup so it doesn't matter too much. Shulk struggles against rushdown characters so I think Squirtle does pretty well in that matchup too, but it could just be even. I simply don't think he loses against Villager and that's all I have to say about that. I also think he does pretty well against Wario because of his small stature (landing up air is less of a threat) and comboing Wario is pretty easy. Wolf does not bother me too much either, he's a fast faller so you can combo him a lot and watergun is lethal against Wolf, even if withdraw is not as helpful due to laser. It is not too far fetched to say that Squirtle goes even with Falco and Fox but I think if anything he has an advantage because of similar reasons talked about above, and against Fox withdraw is really helpful. I think Squirtle destroys Ganon--while you could get hit with a random stray smash attack and just die, Squirtle just overpowers Ganon in neutral and he can get easy money gimps and combos. Theoretically I can see him beating Greninja too for the same reasons I've been stressing. I don't want to make it seem like I think Squirtle wins every matchup, so I'll point out that I think that Inkling kinda destroys Squirtle. He just gets outranged and Inkling's melee tools simply beat out Squirtle's. I know most of my critique of your chart has been in Squirtle's favor, but I think that Squirtle is incredibly underrated... with his great combo game, best disadvantage of the three, solid neutral, and near lagless moves, I think he's a low high tier character, while most others will put him somewhere in mid tier.
I've already said too much, so for now that where I'll finish my input. Maybe soon I'll make my own MU charts for the trio, but I'm gonna spend a bit more time learning the ones I don't know too well yet. It's really hard to rank them individually though, because that's stripping away a huge part of the characters... my above mentioned changes may not be the most accurate only because I consider Squirtle's best killing move to be down b. Even against what I'd consider Squirtle's "good" matchups, I usually opt to end the stock with Ivysaur and sometimes even Zard before moving back to Squirtle to play neutral. However, it's that uniqueness in the character that makes PT have such high potential, and I believe that as the meta progresses PT will only continue to go up in the tier list, maybe even to top tier.