i do not think that an NN3 will work cause of the weight of the D3. You should use a NN5. But try yourself, the lower rail setting for the NN5 are 86 for the lower and and 68 on the upper rail.
The 20mm lenses nnp is according to swissarmyfork 68,5 mm from the film plane = sensor plane. Distance lens flange / sensor plane is 46.5 mm. So measure ca 64mm from the lensflange, mark it on your lens, bring this above the rotation point of the NN3 and read your rail settings.
I suppose the rail settings should be the same for upper and lower rail. Just test it.