Hi, sorry for late reply, just been trying to do the same thing.
You can almost do what you want. Plug the HDMI feed into AV1-4, and the analog connection to the same AV1-4 analog connectors (just right of audio 1 - 3 input as you look at the rear panal). Then turn on the AVR and select that AV channel, this will give you hdmi video and hdmi audio. Press the option button, input settings-> Audio Select -> Analog.
Then you can use the scene or HDMI out button to turn the video on or off (as you said you wanted to assign audio 3 to an AV, assume you also want the video off sometimes!)
I hope this helps!