Let's talk about what specific differences there are between these 3 devices which seem alike.

 Differences between a Bluetooth receiver and a Bluetooth adapter:

The Bluetooth adapter is generally used on a computer to connect to a Bluetooth device, and has the receiving and transmitting function as a Bluetooth signal , which can be regarded as a type of a Bluetooth receiver;It is just like a network adapter is used to connect to a network device.

The Bluetooth receiver is a special device and is generally used with other devices. For example, a GPS Bluetooth receiver  can be connected to a mobile phone using a GPS function. Different Bluetooth receivers have different prices.

Bluetooth adapter

 Differences between Bluetooth audio transmitter and a Bluetooth adapter:

Bluetooth audio transmitter

     Bluetooth adapters can wirelessly send pictures while Bluetooth audio transmitters currently can not send pictures.
     Bluetooth adapters can connect to the Internet wirelessly whereas Bluetooth audio transmitters are currently not available.
     Bluetooth adapters can  print  wirelessly while Bluetooth audio transmitters are currently not available.
     Bluetooth adapters can be audio and video delivery, and you can connect to devices like Bluetooth keyboard, Bluetooth gamepad, Bluetooth camera, Bluetooth scanner, etc.

      The Bluetooth adapter is used on the computer, and has the function of sending and receiving, including voice and file signals. It can be used as a sound card when matched with a Bluetooth headset, and some can also be used as a Bluetooth network card. In short, it is a two-way file transmission device;

      A Bluetooth transmitter  is relatively simple, which just converts the voice signal into a Bluetooth signal and send it out for Bluetooth headset to receive. It hasn't other functions.