When I fight multifaction battles it's usually an ally that's 'helping' me. With 'helping' me I mean when I attack and some other army wants a shot at that province too. I such cases I never engage the enemy fully: I wont let some other faction take some land from me that I could take without help. I won't sit and wait until it's over but I use some of my infantry as strategic reserves and hold my cav back until I know I can disengage quite easily.
If the ally would actually help me, for example when I'm defending, then I would give it my all. But this has never been the case.

My biggest one was when I (English) attacked a rebel held province in order to end a German crusade going through my lands. I was joined by a German force (not the crusade) and a Byzantine force. There where probably about 6000 men on the field. I took brought my archers forward together with a few units of feudal spearmen in order for my archers to take shelter when necessary. The rebels were hiding in a forest that was as wide as their line but it wasn't deep. so I used my light cav to lure some units out and then flanked them with another unit after showering them with arrows. When the Germans and Byzantines engaged I send my cav in behind the rebels and charged my heavy horses in the back of the general while my light horses caught routers. I kept half of my army back but still I felt like I did my part of the battle ( disrupting and destroying their right flank, killing the general and denying them a chance of rallying or sending reserves). A few turns later both the Germans and the Byzantines attacked me, if I had known I wouldn't have engage at all.