General rules of thumb for these sorts of situations:
If you want to aid a friend in defending, you need to be allied to them.
If you want to fight an enemy by attacking, you need to be at war with them.
For this specific instance, I would say that Realm C will almost certainly stay out of the fight (because they aren't allied with any defenders).
Unless when you say "at peace" you actually mean "neutral to" (which, as a relatively savvy BM player, you probably don't), in which case, being set on aggressive, they'll attack Realm A.
This honestly makes little sense. Why would we stay out of a fight, in enemy territory, against a realm we are war at? I'd understand if we were in a region of Realm A, but we're in a region of Realm B. In fact, this makes it more likely for you to attack a neutral realm than the one you're at war with. That is idiocy, it makes no logical sense, from any viewpoint whatsoever.
