I've seen this question before and I suggest that one possibility is China during WW II. Where two chinese forces were fighting a civil war during the Japanese invasion. Both nationist and communist chinese forces bitterly hated each other and so there was all sorts of backstabbing behavior. It wouldnt surprise me if some situations involved all three sides fighting each other simultaneously.
Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again.