NVidia's newer drivers have an option with monitor setup to horizontally spread the desktop across my 2 monitors. I'd imagine the same works with 2 vid cards and 3 monitors (2 are just not helpful for gaming).
I don't always do this. I'll give an example, though. When I don't do it, AH Film viewer has to be on my primary display. If I stretch it onto the secondary or move it there, it'll chug and slow down. If I use the nvidia settings to create 1 big desktop, it does something different with the way the video card processes things, and I can run across both monitors (in AH Film viewer) without the stutters. It does impact fps, though, the larger I make it.
Might try doing that before going for any expensive alternative. It's free (built-in).
EDIT: I looked it up. Right click desktop, "NVIDIA control manager" which I have in advanced mode, "display" tab on the side, "set up multiple displays" sub-section, and it's the option that says "As one large horizontal desktop"