I'd say it would be easyier to plug your PS3 straight into your speakers. I assume they use a minijack input. What you can do is take the standard deffinition cable from your PS3, take the red and white plugs and plug them into a cable like this:
Female phonos to mini jack:
http://www.lsaudiotech.co.uk/ekmps/s...to_mini_sj.jpg
You might be able to plug that straight into your speakers, depends on the design of them. However, if you use one of these you can plug your PC and PS3 into your speakers at the same time, weather you plug this into the back of your PC or the back of your speakers.
Mini jack splitter:
http://files.sandberg.it/products/im.../502-16_lg.jpg
Hope this helps,
Wisey