Depending on what kind of speakers you have, like if your speakers have a two sided stereo plug, or if it is straightwired from the speakers...

If it is a double sided (male/male) stereo jack, just use a RCA to Stereo and plug in the PS3 to the RCA side, and the stereo side back into the speakers...

.. Or if you can find a female RCA to Female Stereo that would work easier..