I’ve run mopidy -v on the PiMusicboxes and the thing that stands out is
If you do that while there is already an instance of mopidy running then I think they use the exact same config file so you can fully expect the second instance to fail to bind to the address that’s in use by the first instance.
I’m currently baffled too. Is it the same version of Mopidy on all devices or are you using the old 0.19 version on the PiMusicbox devices? My only guess right now is that it’s actually the classic Spotify playlist issue that is present in v0.19 and the introduction of the 3rd device is pure coincidence.