You should consider the civil war inBosnian war (in Yugoslavia) in the 1990s, there were basically three factions fighting each other: the Orthodox Christian Serbs (Serbia), the Catholic Croats (Croatia) and the Slavic Muslims (Bosnia). They were fighting each other several yearsThe war was three-sided from 1992 to 1994, and two-sided in 1995-1996. Initially
You can see that even the Muslims and Croats were united against Serbs, but latertable of belligerents in the fighting sparked between Croats and Muslims as wellWikipedia lists three sides.