If you have 3 s3's I would use the EVGA 1600 what g2 it has more PCI-express connectors which you will need to run them. on my S3's Each had its own EVGA 1300 watt PSU but I need one of them for me new pc build so you know what happened then.
That's good for your efficiency. However the more power I supply to mine, makes no difference, they max out. So the draw from each max's out, so I'm able to run a very good quality PSU at a lesser wattage, though I could do with a little more ;-) Always need more power