Are you sure 3 leads isnt too many? I thought 8 pin PCIe was 150 watts max. 3 X 60 is 180. Seems like 2 per pcie lead, is optimal if you want to run dual mode.
It really depends what mode you are running in. For scrypt only mode 3 plugs is actually way underpowered as you can get just about 20 devices @10W on a single PCIe cable.
The 18AWG wire can handle 10A and the minifit pins/housings are rated for 9A. Theoretically you can pull 9Ax12V=108W per wire pair. I would not push it that far but safely getting 80W per wire pair has been done ad infinum by miners. This gives 80W*3pairs=240W per PCIe cable.
Those numbers from the PCIe spec are really safe and are meant to protect the graphics card and motherboard which we will not be using in this instance.