DxO mark does sensor noise comparisons
https://www.dxomark.com/Cameras/Comp..._1061_1020_975
Sports (Low-Light ISO) rating is the ISO at which for a standard low light level, the noise is 30dB less than signal.
The result for the three cameras listed is
D750 ISO2956
D7200 ISO1333
D500 ISO1324
My experience with other cameras is that you can, when necessary, capture at 3-6x the Sports (Low-Light ISO) rating given especially when stacking. My camera has a Sports (Low-Light ISO) rating of ISO1160. But I have used it at ISO 3200 and 6400
ISO 3200
http://www.iceinspace.com.au/forum/s...d.php?t=143911
ISO6400 single exposure with some software noise reduction
http://www.iceinspace.com.au/forum/s...d.php?t=132765
ISO6400 with 4 x 15s images stacked
http://www.iceinspace.com.au/forum/s...d.php?t=146519
and even at 12800 or 25000 providing I noise reduce, downsize the image and possibly go monochrome.
http://www.iceinspace.com.au/forum/s...d.php?t=105063
The D750 is clearly the best for low light & astrophotography of the three Nikon cameras listed above. It also comes with the biggest price tag. The other two cameras are line ball and with higher ISO ratings than mine are very capable of good results.
You just have to decide how much to spend.
Cheers
Joe