I'm pretty sure you can do that with softTH but there is much more of performance hit than if you run off 1 card or Crossfire/SLi.
SoftTH seems like a good solution for older titles like the Race series and rFactor. However I think my (and most people's) system will struggle if you try running it on one of the coming titles (AC, rF2, GTR3, pCars) on 3x1080p mons at max settings (anything less is a nogo for me).