The PSU seems to shut down when feeding it with just 2. It might be because the cables are made for just 75W and it pulls 180W through each (so it drains 360W in total).
So, I'm thinking can I plug a 3rd pci-e cable, without overclocking it just to spread the power a bit more, or it doesn't work like this?
If the PSU is shutting down, either it is overloaded, or one rail of a multi-rail PSU is overloaded, or your AC outlet is not supplying enough power for the PSU. If the cables are overloaded, it will cause them to get hot, but that alone should not cause the PSU to shut down until the insulation melts, and the cables short out.
If plugging in a third cable helps you, then maybe the first two cables are on one rail and the last cable is on the other? Try to make sure each blade of the S3 is on a separate rail if you can.
All that being said, it will not hurt anything, and can only help to plug in PCIe cables to the extra slots on the S3, whether you are over clocking or not.