Didn't work. For five hours. Arg.
I've run dual head for several months now, and it is cool. I recently picked up an old monitor for free, so I figured I'd go for a third head. Digging through old boxes, I found what must have been a 5 year old 3dfx card, cleaned off the dust as best I could, and stuck it in a PCI slot. NOT SUCCESS! I could find no configuration to make this combination of hardware work. Not that this surprised me greatly. It would have been nice if XFree86 were advanced enough to handle this, but seeing as the video cards were manufactured at least 5 years apart and by different companies, I won't hold this failure against X.
That was all a week ago. I ordered a PCI TNT2 after the 3dfx failure. Yesterday it arrived, amidst much excitement and fanfare, and was immediately installed. It worked just as well as the 3dfx, though. So I am unhappy, both with NVidia for not providing better documentation and XFree86 for not working. If anyone does know how to get a two-card, triple headed X configuration working, you are commanded to respond to this with details.