You don't need to plug all four PCI-E connectors into an S3 unless you intend to overclock it anyways. If you're going to overclock, this is a good supply to do it with though, 83 amps to the 12v rail.
AWG size limits your wattage, but from the pictures that looks like 14ga to the connector going down to a short run of 16 gauge. If it's not, this is the minimum you should run in any circumstance.
So since there's only 3 of these would I be ok to run one overclocked using 2 cables? And the second s3 on the third?
Sure.
This really isn´t rocket science, just try it out and feel the connector / cable temperature after an hour or two.
Warm is okay, hot is not.
You can always produce another pcie plug with an adapter (2*molex/pcie), just use two different strans of molex (and an adapter with a decently thick wire).