that's great news!! (not for my wallet, but for myself!) From the description on Eric's Spiel list, I had the impression this was Norse and Japanese only. In fact, there it says:
This first installment of Myths at War contains 150 cards split between the Nordic and Japanese mythologies. Each mythology includes two pantheons (cards that define your game strategy) + 33 cards for the gods deck (which includes characters, events and resources) + 40 cards for the designs deck (actions, equipment and summonings).
So I was a bit on the fence due to the 40 EUR cost. So glad to hear it's GDM I & III combined, with 300 cards! I really hope I can grab a copy!