The f/1.8D is a D lens and has an aperture ring, the f/1.4G and f/1.8G are G lenses and don't. The D lens has screw-drive autofocus, which only works on D7200 and above bodies (in the current lineup), while the two G lenses are AF-S and focus will work on all current Nikon DSLRs, even the low end consumer DSLRs without screw-drive built into the camera (D3xxx and Dxxx).
Beyond that, the faster f/1.4 lens is considerably more expensive. The difference between f/1.4 and f/1.8 is two-thirds of a stop, and that could make a difference to some low light shooters. However, at f/5.6 the optical differences between the three lenses are nearly indistinguishable, so the real issue is how often will you be shooting wide open? Always? Then get the f/1.4G. Rarely? Then get the very low cost f/1.8D if your camera supports it; otherwise, you're stuck with the f/1.8G. Curiously, the f/1.8G is probably the sharpest of the three in my experience.
