I cannot compare the D750 performance, only the D3 and D600, last one has the same chip set as the D750.
The prefabrication of files, also of the RAW, is generally better in the fifth generation, so there might be an advantage at lower iso values like 1600 and 3200 but with high iso like 10.000 and 20.000 the fifth generation leads due to exspeed5.
Also the AF is much better, the WB in difficult light is much better, body ergonomics is better esp with the grip mb-d17.
Speed of 10fps, buffer size if needed, XQD as robust and very fast media. If you own the D750 I would consider the D500 as a second body, not as a replacement.
Why? The files of the D500 feature a clean and clear look, some might call it "a digital look", while the 24 MP full frame chip in 600, 610, 750 has a very subtle magic I cannot really describe. I also see it in the reaction of people towards the pictures. More positive emotional towards the D600 results.
If you would own the D3 I would clearly say: replace with D500. It runs circles around my old work horse.