Or the ‘free for all deathmatch’ approach to history.
Reading about all the unpleasantness in Syria, you have forces fighting for the Assad government, the Syrian rebels, Kurds and ISIS who all hate each other.
How many other conflicts in history have had three or more ‘sides’? Off the top of my head I think the Chinese Civil War/WWII crossover would probably count, you have the Nationalists and Communists on paper both opposing the Japanese but in practice busy fighting each other. I’m sure at some point in the history of the Balkans this has happened multiple times. Any other obvious ones spring to mind?
I think this may need some limits established on conflict size. I mean, if you look at something like the period of the “Wars of Religion”, alliances and sides kept shifting, but as you go smaller things get more delineated.
I would say that something where one part of a side betrayed their allies without joining forces with the other side (like when the Communists massacred the Anarchist leaders in the Spanish Civil War) doesn’t really count, it’s a side of two turning on itself rather than three sides.
The Great African War or Second Congo War (mainly 1998-2003) involved nine African countries and at least 20 armed groups, many of them mutually hostile. That might have been one of the most complicated ones ever.
By the end of World War II Italian fascists and communists had aligned to drive the Nazis out of Italy. I’m not even going to try to describe all the alliances and rivalries in Yugoslavia at that time, but suffice it to say some were in favor of pre-war Yugoslavia, some were in favor of Serbian-led union, and some were in favor of going back to pre-WW1 nationalities.
The Mexican Revolution effectively had three factions fighting for a time, with Carranza opposed by Pancho Villa, a former ally, in the north, and Emiliano Zapata in the south. While I don’t believe their forces directly engaged one another, and they were nominally allies, Villa and Zapata were rivals and united mainly in opposition to Carranza.
Many places have had a period of many-sided civil war, when society has effectively broken down.
England for example had “the Anarchy”, which while the main combatants were Stephen and Matilda, approached a war of ‘all against all’, with all sorts of nationalities and factions clashing (Scots, Welsh, Angevin, local lords with each other, etc):
The Japanese had the much longer-lasting “warring states” period:
This was named after a somewhat similar period in ancient China:
Ethnic Serbs vs. Bosniaks vs. Croats during the Bosnian war, especially around 1993. Many more factions involved, but those did the most 3-way fighting rather than co-belligerents.
Wars of the Three Kingdoms - Wikipedia lists 5 factions, but alliances shifted and if you want to get technical it was at least 3 wars centered on England, Scotland, and Ireland (inextricably linked). At any given time, I think there were no more than 3 factions duking it out.
ETA: and of course a strong religious element, Anglican vs. Catholic vs. Royalist vs. Puritan vs. Presbyterian, and that brought in foreign mercenaries who supported one or more sides.
In WWII, in France, there was the Gaullist Resistance & the Communist Resistance, both fighting the German occupiers, but also doing quite a bit of fighting between each other. This seems to be fairly common when several groups are fighting a common enemy – ‘the enemy of my enemy is my friend’. Even WWII itself, when the Germans invaded the Soviet Union, suddenly the Soviets became part of the Allies, despite fighting a few years before.
This very, very, very ugly war had four combatant nations, although it was one against three. Didn’t turn out too well for the one (Paraguay), which lost the vast majority of its adult male population: Paraguayan War - Wikipedia