Try monoprice.com - they have all sorts of adapters/cables.
these might be what you need
http://www.smarthome.com/97350/6-Foot-HDTV-VGA-to-3-BNC-Video-Cable/p.aspx
http://www.electronix.com/catalog/product_info.php/cPath/406_409/products_id/12434
also are you sure theres only 3 bnc connectors on the system? not sure if newer monitors support sync on green.
