Is there a Pokemon evolution line that fits the Sphinx's riddle?

There is not yet any Pokemon who change himself physically or who evolve in such a way that it would answer the Sphinx's Riddle. There is some who are starting with two legs and who have three, then four after evolving (such as Roggenrola and its evolutions), but not in this specific order.

Given the answer to the original sphinx's riddle, you shouldn't look at this so literally, especially the "three legs" phase. Consider this:

Slakoth is clearly on all fours, much like a human baby. Vigoroth follows, proudly standing on two as a man in his prime might. Slaking has, however, reverted to a three-limbed stance, just like an elderly man with a cane.

Rhyhorn is close too. 4 legs as Rhyhorn, 2 as Rhydon, and then 2 again as Rhyperior but with a bulkier, rock tail that kinda could be a third leg. Ie the 4-2-3. That’s what I’ve thought of as the closest but the slakoth line works well too.

It took years, but yes, we finally have Sobble. Before now the closest was Chimchar; Monferno is bipedal and Infernape sometimes does the three-point superhero-landing stance.