Are we talking young or full-sized rancor? Young rancors are small and vulnerable (i.e. KOTOR) but full-sized rancors are very, very tough (i.e. Jedi Academy).
In either case, the wampa would get clobbered by the acklay or the reek. If it's a young rancor I'd say it could take out the nexu but then it too would be beaten by the reek or the acklay. If it were a full-sized rancor, then it would beat everything.