Denon support got back to me with the following:
Thank you for contacting D+M, we value you as a D+M customer and appreciate the opportunity to be of assistance!
Zone 3 only supports analog outputs. HDMI sources cannot be passed to Zone3. Also in order to analog sources you will need to assign analog for the input assigns for the specific input you want to use.
If you have any further questions please feel free to contact us again.
Thank you for contacting D+M Customer Support. Have a great day!
Thank You
D+M Group
Customer Service and Support
Apparently Zone 3 does not support digital sources, but I don't see that mentioned anywhere in the specs or manual.
I'll try it out tonight to see if I can get any analog sources to play out of zone 3 pre-out.