Pictures of my "mining-rig". Not as big as yours but..... 
I see you have the Corsair 750M for 2 S3. I do have the Corsair CX600M with 2 PCI-e (on 1 cable) for 1 S3.
Question: Are your cables "hot" ?? I noticed that mine get pretty warm just with 1 S3 hooked up to it. NO OC!
Just wondering ...
Nope they are OK, but I have them also without oc just default frequency. I don't want to buy new PSUs
Hmmm ... So why are mine "hot" ?? ... Wired ...
Well, I ordered an 8 Pin to 6 Pin connector. It may help. We'll see.
Thanks
yes. along the lines of what lightfoot said. each conductor(wire) is rated for so much current and wattage without warming noticeably. as you draw more than that they warm and the interaction Lightfoot mentioned starts to exponentiate.
the 8 pin to 6 pin will help. what type did you get though? i think the 8 pin youre thinking of is an EPS connector not pci-e which has a different pinout.