A bit of further pondering led me to suspect that I kind of danced around your question in my last post... if I'm understanding it right, your question is not "what happens when Group C attacks Group A," but rather, "is it legit for Group C to attack Group A."
But I still maintain that the rules as written already cover this... if it were not legit for Group C to attack Group A, then there wouldn't be rules for the consequences of the attack. Because the rules specify what happens when there's an attack like the one you describe, they tacitly endorse the attack.
just my ever-so-humble.
