The problem too is that no one knows how strong the others actually are. Land doesn't mean anything, and nor does the amount of faction members as some aren't as active as others.
So let's split the map in three, and have three captains pick teams. Then we'll split all the mercs the same way. Then we'll have a very equal three way war. Whenever two factions battle, the third will even out the rosters to ensure maximum casualties on their enemies.
There you go, problem solved. Please show your gratitude with gold and looms please.