I was doing something similar. Grab a flavor of vnc for each unit and intall the server on each box that wont ahve a keyborad & mouse attatched. On the one with the keyboard and mouse, install the client.
Then you just run the vnc program and you get a window for each remote system. X11 based GUIs generally will give you a new Xsession. I dont know about OS X. But winders will give you control of the unit as if you were in front of it.
As long as there is a LAN connection, you really arent limited to how many connections (excpet bandwidth). I highly reccomend that you close out any sessions that are not going to be used -- sending 3 1024x768x32 screens every refresh cycle can be quite exhaustive