An S3 uses only like 340Watts so 500W PSU should be sufficient. However it probably only has 2 PCIe connectors.
I am assuming that the heartburn people have with the OP's statement is that the post appears to imply that two S3+ are being powered by one 500W PSU.
I've got a couple of S3+ units and a 500W power supply (been running them both for a month or so) that I'm selling, if anyone's interested.