With light and dark as the first factions, the third one should be neutral or balance.
It could be all the "Nature" related races.
Some ideas borrowed from other games : beastmen (based on wolf or lizard, aka dragonkind), sylvian elves, faeries, elementals... Even orcs, goblins or humans can be neutral with the appropriate lore, they can be great mercenaries, only fighting for gold or fame.
My 2 gold pieces.