Fortunately, the solution IS simple and any receiver with a zone 2 capability will work. As far as which particular one would be best, that will be up to you and depend soley on what features, power, cost, etc you are looking for.
If you are OK with zone 2 and zone 3 playing the same source (but still independent of zone 1), then you need impedance matching volume controls before the speakers in zone 2 and zone 3. In-wall volume controls are nice because you can always mute one of the extra zones if no one is in that room to listen.
You effectively run zone 2 and zone 3 as a single zone being powered by the receiver's zone 2 speaker outputs by wiring the two pairs of speakers in parallel. You can do that by simply twisting the wires together or by buying a fancy in-wall plate that does that for you internally (it has 2 inputs just like a normal wall plate for two speakers but has 4 outputs because you are connecting 4 speakers). You must use impedance matching volume controls unless all speakers are 8 Ohm and the receiver is certified for 4 Ohm loads.
An alternative to using IM volume controls is to purchase a speaker selector but then you've got another box to put somewhere.
When using the receiver to power zone 2 (which in your case is really zone 2 and zone 3 acting as a single zone), zone 1 will be limited to 5.1. If the receiver also has zone 2 pre-outs (and many do), then you could retain 7.1 in the main zone and still have zone 2 by connecting an addtional amplifier (which could be another receiver) to the zone 2 pre-outs. The downside to that though, is turning on the amp and yet again - another box. There are ways around the hassle of having to turn on the zone 2 amp such as macros on a universal remote or if you're lucky you find a receiver that has a zone 2 trigger.
