Even if Steel, Ground, and Rock were a perfect type triangle, I still don't know if I'd be a fan of it, as there are many Pokemon out there that are dual-typed with 2 out of 3 of these (ex: Steelix for Steel/Ground, Aggron for Steel/Rock, and Rhydon for Ground/Rock). And I've never been a fan of either Rock or Ground type Pokemon, so I'd probably just go with the Steel type starter if those were my only choices.