It is tyipcal AI behavior, try working it with teams locked and set groups

On a few occasions (that is, infrequently) you may see AIs break their alliances with each other. It usually follows after another AI has been defeated, and since AIs always need an opponent to fight, they choose one of their current allies to go to war against.

I've only witnessed it happen twice in all my times playing Sins, first occasion was when myself and two other AIs were left, one AI ended up utterly destroying the other's fleet, and the second, more recent occasion, the two only fought for about 10 minutes before they made up and became allies again (though this was enough for me to step in and steal a few of the planets they lost from each other).

Good advice, but another option is to make a map with multiple star systems, and have the first star with one player, and have the other stars with two. That way, they will probably be busy fighting EACH OTHER, and not you; this usually works until they start expanding to other star systems, at which point they might ally, but the two AIs in system B (you are in system A) probably won't ally with the AIs in star C, so you just get a ceasefire with one of them, keep them happy so you keep the cease fire (you should get a peace treaty with one AI,but other than that, don't get a peace treaty, they are harder to break, and when you do, you can't attack for a while, and whatever you do, DO NOT GIVE THEM planet or ship vision, it just gives them battle maps of you defenses for when its just you and them), and then you both attack the the other two AIs, and this is where you can use the peace treaty to use attack pings. Then Once Star B is subjugated, then you stay at peace with the allied AI in Star C, and you both conquer the other AI, and then wait a little while, build up a MASSIVE fleet, then if you aren't back at war already, go to war and go on the offensive immediately. Use superweapons when necessary, the Kostura works best, and you should win pretty quickly 

Do their missions, as much as possible.  Unless you are good enough to win playing 1-v-9, you can't ignore the diplomacy aspect of the game (even if you aren't playing the diplomacy beta).

Impossible, if certain situations arise, such as a destabilization of power that results from the defeat of one attacker which results in a shift of alliances, which results in more fleets attacking your planets.

OR, you could give in to their demands w/ skilled diplomats tech for TEC, and offer them cease fire and break it when necessary