In a 2-way Crossfire HD 7950s consumed 279W and in a 3-way Crossfire HD 7950s consumed 411W: http://www.guru3d.com/article/radeon-hd-7950-crossfire-... Add another 100-150W for the rest of the system; the HX750W is electrically sufficient for a 3-way Crossfire. The Corsair CMPSU-650HX that is made by Seasonic and has four 6+2pin PCIe cables is actually sufficient for a 2-way Crossfire of HD7950s. The GTX 660ti has basically the same power consumption....if the PSU works for one, then it is good for the other.
Edit: Changed a reference to SLI to read Crossfire.
The CMPSU-650HX's capacity on the 12V rail is 624W. Assuming the rest of your PC consumes 150W (unlikely), then the full load power draw (279W+150W = 429W) is under 70% of the HX650: 429W / 624W = .6875 or 68.75% Operating a quality PSU at this load is imminently safe and is actually in the typical PSU efficiency 'sweet spot' between 40-70%. If the rest of your system uses 100W (more likely) then you hit around 60% capacity of the PSU at full load. Again, that is a very safe range to operate your PSU.