I did a little digging, and both here in this help article and in some user reviews I’ve read online, the 1000XM3 only supports playback and connection from one device at a time.
WH-1000XM3 | Help Guide | Pairing and connecting with other Bluetooth devices
Register (pair) a Bluetooth device with the headset.
It will pair to multiple devices, but only supports one active connection. So it looks like Bose still has them beat on that front!
I’ve never run across a Bluetooth headphone that supports more than 2 simultaneous connections. In fact, the Bose is the only model I’ve used with that feature. I’m not sure, at this rate, that we’ll ever see a Bluetooth audio device that supports the 3–5 you’re looking for, since Bluetooth audio is mostly for personal listening on headphones or a speaker…but hey I could be wrong!
Thanks for reading.