I don't know if there's a limitation to 75W per PCI-e cable. In my case there would be ~120W per wiring. Do you have a source for the 75W limitation? I'll look into the PSU reference guide.
I have been searching on Google and the general consensus is that a 6pin PCI can easily handle 150W. The difference between the 75W 6 pin and 150W 8 pin is just 2 ground wires. Some people even put the 6pin cable into the 8 pin connector of a GPU without problems.
Correct, the specs of the connector state 13Amps
per PIN.
Just check that the cable gauge connected to those pins is sufficient, but 150W should be possible with any of them.
So yes you can run 3 modules off one 6pin plug. I think i'll go for a ~850W, gold rated, single rail PSU perfect for one tower of 16 modules.