Those two are all you need.
If your PSU can do dual 8 pin CPU, then by all means do it, but you don't need it. The extra one is there because it might be helpful with pumping 1.8v to your CPU at 6+ GHz under LN2.
The extra two 4 pins for PCIE power are unnecessary except for maybe high end 4 GPU set ups that are heavily overvolted. Use em if you want, it won't hurt anything, but you ain't gonna need em for your setup.