If nvidia says its not a good idea then I'd take their advice. With more than 2, you might also cut into the maximum PCIE 16x bandwidth in which you get bottlenecked and the 3rd doesnt give an equal performance return as the 2nd did to the 1st video card.
If it was my build, I'd go with 2 video cards vs 3. Unless your going to be crunching data with the GPU's vs gaming like Folding@Home etc, your going to find that you likely will have plenty of GPU power with 2 and anything greater wont be noticeable performance gain in games and will be a waste of money in both the 3rd card as well as greater electric bill to run the system.