The game currently doesn't support 3 way fights. The battle system would have to change so it can support it.
Ideas : instead of having two sides, you would have the first two 'first' sides fighting each other and having the third one coming from the side. Or, to keep a closer "layout" compared to what exists now, from the back of one of the nations involved (the 3rd attacker could chooses which one ? as a bonus ?
). Or something else.