Just like with the Straam and the Mak, this is something that's mentioned early in the books that KAA doesn't really do anything with. As well as Alloran mentioning Yeerks being on the homeworld in book 8, and one Andalite saying he fought alongside Hork-Bajir in the war. Maybe the third race is the Onganchics. They have no homeworld, and one of their ships were taken by the Yeerks before they found the Hork-Bajir. That could be the third race. Still, it's not much.