Before Gods, there were not fully evolved human species. Therefore Gods came and created 3 races. Due to universe's own laws and dynamics, there has to be diversity on each habitat. Gods do not act against natural laws. If things had went well before their arrival, these 3 races would be living in here, in their own habitat, evolved accordingly to their environment.
As for as Whites being direct descendants, it's only because Whites are, since they would had been evolved in Europe, would be alike to Nordics, and therefore Nordic Gods (including Zeus/Satya) gave them their own genes. But for Asians and Blacks, I believe, there could be other alike-alien species on the universe, more evolved and very advanced. I believe they might had gave them their own genetical gifts.