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