This can be done in about 10 turns on easy difficulty. You already start with Athens as a client state so you only need to subjugate Sparta and Epirus. Playing on easy will make factions less likely to betray you.
Disband fleet, recruit units in both armies. Attack Epirus' first settlement with your closest army and recruit a bunch of mercs if you need. Force march 2nd army to catch up. Move towards their 2nd settlement, recruiting/hiring along the way as needed. Then once you take out their remaining settlement, subjugate them. Then march towards Sparta and again hire mercs if needed but should be able to just AR win against them.