PeridotGX
Smash Hero
Huh, I've never really thought about that. To be fair, for all intents and purposes we have plenty of three stage mono lines, because of how weird Game Freak has treated flying. The Galarian Zigzagoon line is also essentially pure dark, because the addition of normal just turns a weakness x4 and a resistance to an immunity and that's about it.It's a little weird how after 1000 Pokemon, we still don't have a pure 3-member family from Flying, Poison, Ground, Bug, and Dark types.
If we wanted to fix this without inventing wholly new lines, I think Swalot, Sandslash, Spidops, and Theivul could get away with monotyped evolutions (due to the dearth of mono flyings there really isn't an option there unless you want to give corvesquire a branch evolution.