Again, assuming that each S3 draws 390WAC at the wall, the EVGA SuperNOVA 1300G2 would give you around 19% headroom (81% load) with 3x S3s.
yes, but if you truly need 4 connectors per S3 for a mild overclock to 390W (I doubt it), then you need to have 4 more connectors (EVGA has 8 total). I solved it by ordering splitters: there is an awful lot of discussion whether splitters with 16awg wires are needed or 18awg is enough. According to this: http://www.stayonline.com/reference-circuit-ampacity.aspx each wire can carry 7amps, so for three wires it is 21ampsX12v=252W carrying capacity for 18awg wires in PCIe splitter. there is a debate how much wattage can plastic connectors take, but at least 18awg wires seems to be able to take it.