I did some tests on Multi Room synchronising about 9 months ago. My conclusions at that time were that satisfactory results could be achieved with SIMILAR RENDERER HARDWARE. I had 3 Apple Airport Expresses working with acceptable sync in 3 rooms. When I added the Apple TV to this scenario I had a lag with Apple TV vs Apple Airport Express.
I didn’t have multiple RPI equipment at the time so didn’t do any tests with this hardware, I could resurrect these tests to include RPi’s. Sonos equipment is good at syncing but they do this by restricting the Hardware options (3 speakers possible I think). In the UPnP/DLNA we will have thousands of possible music renderers.
On the DLNA side I also tried an RPI + Volumio, a Sansui streamer and a PC. The results were not synchronised and pretty annoying
The best software that I used for Multi Room was Jriver for Windows. It is quite expensive but there is a 30 day free trial so worth a test. Older versions (which might do the job) V17 & V18 might also be available as free software by now. It had Zone Linking capability and a solution for adjusting the lag time. I didn’t find this reliable enough to pursue at the time.
Regarding point 3. You say you would prefer a Windows Controller. Would this be laptop, tablet or phone ? If so what OS (e.g. Windows Phone, RT, Full blown). I quite liked the web browser UI with JRiver, did the job.
Do you have enough “similar” receivers to get 3 rooms off and away. I’d try out with JRiver Link Zone Function and see how you get on.
I have more RPi’s now so I might have another go with these. I’ll have a look at MusicBee too as I am not familiar with that.
Good Luck and let us know how you get on.