That's a hard lesson you've learned in that game
When playing vs AI the best option is to make allies (by fulfilling missions) right from the beginning of the game. I don't know the Game Mechanics but it can be that AI teams up against the weakest player (which you might have been since you've only had one planet). Anyway starting late game to try to break an AI alliance might be a little too late. Since the missions in the early game are quite easy ("give 1000 credits" and stuff like that) I would suggest to point out one of the AIs to be your ally (by fulfilling their missions) and one AI as "enemy of the state" (putting bounty on his head and attacking his colonies). Best case the later missions of your ally will be to destroy ships and structures of your designated enemy, than you'll be able to kill two birds with one stone.