What does 'pair with at most three receivers at one time' mean? Where do we put the main device?
I think what winigar is referring to is a multi-point bluetooth. Much like Motorola's H710 headset for cell phones. One can 'pair' the headset with more than one device i.e. cell phone, laptop etc. Pairing is the security feature of bluetooth, so the devices have to be 'paired' in order to use each other. The H710 can do two devices, apparently the unit for the Nikon can do three, so you could 'pair' the camera with your smart phone, your laptop and a friend's laptop all at the same time and share images from the camera. Looking at the EasyTagger page for Nikon, it looks like the device attaches to the flash hotshoe and one uses a data cable plugged into the data port of the camera, the bracket pic that winigar attached shows the mounting.