Lose the HDMI cable. Unless you can run it to your receiver, you're not going to be able to utilize the HD codecs anyway.
Get a component (R,G,B) video cable for your PS3. Run that into the component video in on your Onkyo. Then, run digital optical (Toslink) to the receiver from the PS3.
When that is done, run another component video cable from the out of the receiver, to an in on the TV. Change the TV to that input, and you should be good.
So, two cables from the PS3 to the receiver (video, audio). One cable from the receiver to the TV (video).
Let us know how it goes.