Solved! can I connect a wireless headset to 2 pc's and a mobile phone simultaneously

Solution
You would have to connect the audio output of the PCs and phone to a stereo mixer with at least three stereo inputs.
https://www.bhphotovideo.com/c/product/331962-REG/Rolls_MX42_MX42_Mini_Stereo_Mixer.html
or you can make one
The output of the mixer would connect to a bluetooth transmitter.
You would have to connect the audio output of the PCs and phone to a stereo mixer with at least three stereo inputs.
https://www.bhphotovideo.com/c/product/331962-REG/Rolls_MX42_MX42_Mini_Stereo_Mixer.html
or you can make one
The output of the mixer would connect to a bluetooth transmitter.
 
Solution