All three codes describe diagnostic venous studies of the extremities.
Code 93965 a Doppler wave form is similar like 970, 971 but it provides more detail, including waveform analysis, which is going to provide a more in-depth info as to what is happening, like how the blood is flowing from vessel to vessel, in other words, a physiological testing of viens. Basically, the physiological testing are things like pressure measurement,Doppler wave form analysis/technology, without imaging.
Where as 93970, the duplex scans are the ones with real time imaging in addition to wave form; the duplex scanning includes common femoral superficial proximal deep femoral greater saphenous and popliteal vien of the lower extremity.