It is not necessarily the case.
@peter will be able to answer that for ST3’s internal support of multiple BT devices. Other users probably already do what you’re asking, in terms if using a BT MIDI device.
The best I can do is show you that I can have iRig Blueboard plus Bluetooth headphones working at the same time. They do both work within ST3. The only thing I can’t directly verify is whether two devices set up within ST3 would work together. Usually you’d encounter issues only when the devices perform the same function, eg. two BT headphones at the same time.
Here’s my system bluetooth devices showing that you can connect multiple BT devices at the same time:
