You dont really need a switch, thats what the source button is for. Well its really confusing what you are saying, this is what i think: You have a Tv and you have connected your PC and PS3 to it and you want surround sound. Well your going to have to buy speakers and a receiver, or there are home theatres in a box you can buy. If you can be more specific about what your saying that will help.
It is not clear what you are asking:
- does your TV has SPDIF "audio out" (coax or optical)? That is your chance to have audio pumped into a receiver / soundbar
- does your TV has two HDMI inputs? If not - get a simple HDMI switch to choose between PC and PS3.