So how come we aren't thinking about the possibility of all three Pokemon being fully evolved when speaking about the Legends Trainer possibility? I mean, Rowlet, Cyndaquil, Oshawatt, Dewott, Quilava and Dartrix aren't bad either, but at the same time, nothing is also stopping them from doing Typlosion, Samurott and Decidueye. And all three would be different from each other.
-Typlosion would be the speedster of the three with a lot of rolling moves like Flame Wheel, Rollout, Gyro Ball, etc.
-Samurott would be the bruiser of the three, with a decent middle range thanks to it's swords.
-Decidueye would be the zoner of the three, with a focus on ranged moves and maybe a couple trap-like moves, as the arrows from one of it's moves apparently prevents the opponent from moving/fleeing.
That aside, if the Trainer were to have two of the starters in the first and second form, I think it should be Cyndaquil, Dewott and Decidueye to differentiate them from Squirtle, Ivysaur and Charizard. And I feel like Oshawott would be a bit too similar to Squirtle stat-wise. Dewott would have a bit more bulk to them.