Your MB manual will verify the layout of the slots. And it will also tell you which slot needs to be populated first for best performance. I would stay with 2 way SLI in the 2 PCIe X16 slots. The only slot that would be practical for a 3rd card would be the one closest to the bottom of the MB. But that presents problems with accessing the FP headers because it will be covering them.
However, if you are gaming across 3 monitors (5760x1080), the 3rd card will help with performance.