well i got one for £2.50 delivered from a good website but it didnt goto plan.
the pc is pushing more power to the speakers so when ps3 is connected the PC audio works, if pc is off then ps3 audio works.
there goes the idea of simultaneously running PC and PS3 to same speakers LOL
The output stage of the PC sound is grounding the signal from the PS3 when on as PC outputs are capable of driving headphones and so have quite a low self impedance. You need a mixer to do what you are trying to do, there is even the potential to damage the equipment involved when connecting them together as you have here, though most equipment is well protected.