Can someone tell me which connectors on the back of the cabinets I need to use to connect a 2nd expansion cabinet. I currently have 1 expansion cabinet installed.
Also, do I use the same highway cables that I used for the 1st expansion cabinet?

The HiPath 3700 series is a 1 or 2 cabinet system. A left and a right (top and bottom) cabinet. There is no centre cabinet. The system can't be expanded past 2 cabinets...
I might be wrong, it has been a few years since I worked on them. Look for another main cabinet somewhere, this could be 2 systems.