you have to go into the onkyo setup menu and enable hdmi video out.
The hdmi output is set to off as default so connect the onkyo to the tv by composite out from the amp.
The first time you connect your ps3 to the reciever, you will need to hold the ps3's power button in for 5-10 secs when you turn it on. This 'pairs' it with the reciever instead of the tv.
Hopefully this will resolve your problem.