Thanks for the council!
I tried to find the reason why this is not possible. I reset three identical HC-50 Bluetooth units to their original state. With AT commands I checked that all settings were identical, and where they were not, made them identical. Only for one parameter this did not work: the addresses of the BT units can not be reset, they look like 14:1:251744 and are different for different units. Apparently this is what the receiving Bluetooth checks to decide if another unit is involved.