I have a feeling that the 3 cultures are not at all based on three historical civs. I see it broken down as
1.) Gods- all Major and minor gods, also demi-gods..
2.) Mythilogical creatures - goblins,trolls, dragons, unicorns, minituars ...
3.) Humans?The 'civs' would be maybe like different races that can select the gods they worship and can capture or make allies with creatures. You would start by creating a temple or altar to attrack the attention of magical beasts or gods and create shamans or witches to help gather the gods favors or even controll other creatures. You could be race or elves that could have magical powers and raise certain creatures that would inturn appease certain gods.
It does not have to be True Historical content! Does it?