I am working on a very similar setup for my basement, and recently posted a very similar question (see my post titled "Can the 990 do this" in this forum where gonk really helped me out). I am currently combining an NAD T762 AVR with an Outlaw 970. The set up is very complex, but achieves what you are looking for (plus provides AV swithcing for my Zone 2).
The AVR serves as a 4 channel amp for the HT (I am using a phantom center), and I use the receiver to power Zones 2 & 3 using the A & B speaker switch (fed by analog connections from the various sources). I can optionally use the Zone 2 feature of the AVR if I want to send different sources to Zones 2 & 3.
If your Denon has A & B speakers, then you may not need Zone 2 on the 990 at all (just send the analog connections directly to the Denon, and use the Denon to control Zones 2 & 3). This should work fine, unless you want to play different music in Zone 2 than in Zone 3. However, if your Denon has a Zone 2 feature, then you should be able to accomplish true 3 zone functionality.
Just don't expect anyone else in the house to be able to operate the system.