Well, in regards of "power of three", it would make more sense to have a fairy with the power of afternoon?
Eos means Dawn
Selene means Moon, can be associated with the night
So next one would be related to Afternoon or Evening, I guess :P

Third fairy! Now with the power of the afternoon!

I still don't think a third fairy would fit current scholar lore, though. But if there's some advanced class system that goes beyond the soulstone for level 50+, I wouldn't doubt that.