Provided you're running Vista or above, you shouldn't have any problems running two different cards to get three screens up and running. Obviously, you know you won't be able to game across all three, and as Spot35 said Nvidia will disable hardware PhysX due to the presence of the AMD card, but just for running screens you should be fine.
I briefly ran an Nvidia 460 alongside Crossfired 7970s with hacked drivers to enable PhysX. I tried connecting a monitor to the 460, and it worked perfectly. The only issue I had was Just Cause 2, for some reason, would only launch on the single Nvidia powered screen, not the main AMD powered one(s). No idea why, but it was a pretty hacked about setup, so may not be something you'll experience, and it never happened on any other games, just that one.
I'd say try it, it should work. If you find any weird issues and they're a showstopper for you, you can always just take out the card and you've lost nothing as it didn't cost you anything to try it.