When one thinks about it, the idea of three sapient races on one world is not all that unreasonable actually.
As well as the "progenitors made it so" concept that seems to be the case here, the idea of multiple sapient races on one world also works within the context of naturally evolved races as well. After all, here on Earth we actually have a fair deal of creatures who are really close to being sapient (e.g. chimpanzees, gorillas, dolphins, crows, parrots, etc.), so its quite possible there could exist other worlds in the cosmos inhabited by more than one native sapient race.