I had a triple monitor setup working in Windows 7 with an ATI card as my primary and a cheap nVidia card as my secondary. The ATI card powered two monitors and the nVidia powered the third. Everything was working fine.
Then I rebuilt. Now I can't seem to get all three monitors activated to save my life. I don't know if I need to uninstall and reinstall drivers in a certain order or what - but the key here is that I had it working for months. I don't recall any problems making it work originally, so I hope it's just something I'm overlooking.
EDIT: Well crap - I decided to try one more time to enable the third monitor ("extend desktop to this display") and this time it worked - go figure. I'll leave the question open for a bit to see if anyone can contribute ideas for proper implementation using different manufacturers.