DSM2 is the operating protocol established by Horizon Hobbies for the Spektrum brand R/C.Other brands such as Futaba and JR cannot work with DSM2 protocols.
There are some brands compatible such as Orange, Lemon and Deltang that are compatible.The Deltang TX2 modules I and other builders employ will work any DSM2 R/X.
The DSM2 can be set up so that it works in a different fashion depending on requirements.
The DX3 is a Spektrum car controller. It is lot different to the regular stick radios such as DX4 and DX5e.
DSM2 has morphed into DSMX which is now the uprated standard for TX's in The EU
Rod, DSM2 is extremely popular in its 'Air' version. The 'Surface' and 'Marine' versions are much less dominant, have fewer receiver options in the market and they can be much harder to bind with my receivers.
The MLP4DSM/'Blade' transmitter is normally easy to find on ebay and is compatible with 'DSM2 air'. It would be a good choice although limited in range like my Tx2 product. My Tx2 allows you to make custom transmitters and many cheap joystick transmitters can be a donor for high quality joysticks.