What audio output options do you have setup on the PS3, it should autodetect all the correct settings, such as
If that is correct, I would suggest your speaker placement/room setup is not correct or you have poor quality mains/surrounds compared to your center speaker. What speakers do you have hooked up to your system?
Fortunately you are on the right site to get all the information and some of the best on the internet you need so head over audio theory, setup and chat subforum.
What TyrantII says about 3D positional audio is absolutely correct.