thr is no such thing as D2 in prosmatic its only 2
it engages ONLY the second gear. u can use it to get off the mark slowly, required in slippery conditions, eg snow, or can be used for mild engine braking. second gear is really long so u want get much engine braking
D3 and D has same power, but D3 uses only first three gears, hence rpm stays high always, more engine braking, less downshifts to accelerate.