I want to start writing a war-based story. The 3 groups in my story could be the official Government, a Revolutionary/Rebel force (who are "good guys" nonetheless), and the bad guys. Alternatively, it could be 3 groups fighting for control of some common resource (or chasing a MacGuffin), but who worship a different God each.
The key element in either case is there is a strong mutual hate between the 3 groups, and cooperating with either of the other two is a religious/ethical stigma. In other words, two groups teaming up against the third is ruled out.
I was wondering how realistic such a 3-way war would be? Can such 3-way hostilities be sustained for long? If two sides are already fighting, is it even possible for the 3rd group to join the party, fighting both at once without teaming up with one side or the other? Are there any other considerations or tropes I should look for when setting up the plot?
I looked at the Second World War as a close enough real life reference, where the Nazis and friends, USSR and friends, and US and friends had similar sort of mutual distrust (to put it simply), but the US team and USSR team ended up teaming up (sort of).
The other real life reference closer home (timewise) is the complicated situation in West Asia, but that involves n-way hostilities with variable n, and dynamic teaming, so doesn't give me much insights. :-)