I'm not familiar with a "standard audio video 3 prong RCA "
In my previous post I had assumed that you were running some sort of digital signal, either through a coaxial connection or toslink, from your DVD player to a digital input of your processor. I think that assumption may have been erroneous.
Now, when you say a "standard audio video 3 prong RCA " are you are referring to a cable with red/white/yellow RCA connectors on each end? If so, that yellow is for composite video and the red and white ones are for the right and left channel analog audio. You're not getting 5.1 digital sound.
Depending on how and where you're hooking them up, you're getting the lowest common denominator video and two channel stereo sound, which implies no center channel unless you select DPL on your processor.