Quick update: I copied ATI5000Controller.kext, ATIFramebuffer.kext, and ATISupport.kext from that ATI-backup folder to \System\Library\Extensions, and rebooted. On the reboot, I got stuck at the gray screen with the apple logo, and a spinning gear. Left it that way for about 10 minutes and didn't get anywhere, so I aborted.
Any ideas on how to fix this 3rd monitor issue? Thanks again guys.