Colin
As you are using Airfoil I guess you are choosing AirPlay to stream to your three Mu-so systems. But the same limitation applies to Bluetooth. Neither of these inputs to one Mu-so can be used as the source for Naim Multiroom streaming to the others. Sadly, a fact not well explained in the advertising nor always understood by dealers.
Any one of your Mu-so systems can be used as a source for Multiroom if streaming iRadio, Spotify (Connect), Tidal, or playing from a UPnP server. In the Naim App on your iPhone, select the source Mu-so as the "Room" and touch the bottom bar which shows one of these sources as playing. This will bring up the "now playing" screen which will show the four squares Multiroom icon to the right of the volume control bar. Touch this and you can then choose one or both of your other systems as clients.
There is a configuration option for each room that allows you to show the volume control in the bottom bar on the main screen for that room. Naim have chosen not to show the Multiroom icon there however, forcing the extra step of going to the now playing screen.
Naim do not allow you to choose permanent groupings so the selection of clients will be forgotten when, for example, they go into standby.
The analogue and digital inputs are not supported as Multiroom sources either. The only local input that is supported is the USB input. Many, but not all, iPhone/iPad apps allow you to play audio out through the dock connector through this USB input and then select Multiroom clients to play in sync.
Roger
