[Lazy_Perfectionist]

300 provinces at least? Sounds a little high, but you are playing as a team.

For fun, worthy heroes makes an interesting mod. There's also an epic heroes mod you might consider, though it's not popular with competitive, non-team games.

I suggest looking into modifying one of the pre=designed maps (documentation available in dominions directory, and on CD) so that your three nations start fairly close together, rather than on opposite sides of the map.

Not precisely a way to set another nation as allied, but their are a couple of workarounds to minimize battles.
1). Sailing nations (LA Atlantis)
2). Flying nations (Caelum)
3). Stealthy nations and light patrols/pd (Shinuyama)
4). Use castles/forts as doors. If the local forces are set to defend the castle, (not patrol), then they will not engage an invading force. While they're there, they'll have partial control of the territory, but they won't fight you. They will engage any PD. When the 'friendlies' move out, territory will revert to the defenders' nation.