Since the SB-600's do not have SU-4 mode built in, I assume you want to use CLS Wireless Flash Control to trigger all of the Speedlights. I would recommend adding a third SB-600. 1. You already know how to use it. 2. It is fully compatible with CLS. 3. They are available for well under $300.00.
You could get a cheaper Manual Speedlight with an optical trigger but you would have to get two Nikon SU-4 (approx. $75.00) each to trigger the SB-600's optically. Your SB-700 has SU-4 mode built in.