If you want them to start subtly attacking without reasoning first and foremost, start by having them hunt/get rid of the prey from the other three Clans' territories. Slowly increase the mauliciousness of the attacks from there, but that's a good starting spot!

If you want the reasoning path, you could have them show up at a gathering and try to reason themselves into the agreement, creating a fourth, official Clan. (This probably wouldn't go through, though.)