There is no need for three computers to route audio to different speaker locations. Many of us are running 8 to 10 speakers and in some cases over 50 speakers from a single computer running Hauptwerk. Read the Hauptwerk User Guide as a starting point and then begin building a system.
Most of us find more value in using the HW audio groups for output allocation based on cycling audio to multiple speakers for added clarity. It is also possible to route to multiple physical locations such as left and right chambers for theater organs and the physical locations(high, low, rear) that you describe. With enough speakers one can accomplish both methods. Keep in mind that setting up audio groups by division is wasteful in the sense that some speakers will be silent based on which manuals are played. Still, if the physical locations add realism then that's a valid approach.