Do you use RYW cables in conjunction with the HDMI? That's what I do.
Since you are saying it works fine with your PS3 having the HDMI solely, try restarting the computer, plugging in the cable, and if you are able to, change the source channel to something like AV2.
Tried restarting the computer and plugging it in. Even tried keeping it in and restarting. Both didn't work.And how do I change the source channel?