have you tried the automatic configuration of X?
That scans for graphics devices.
If that doesn't help give us your /var/logXFree86.log
Maybe the problem is that you have 3 equal graphic boards in the PC.
I believe to remember that ATI once stated that more than one board of each type is not supported by them. Maybe it just doesn't work. Or have you tested them in Windows? - If you wanna test it try 9X. The Mach64 doesn't work as secondary card on NT based OSs.
Maybe you can find some more hints here: http://www.realtimesoft.com/