I recently sold my D3 and my D600. Both went for roughly the same price. The Image Quality of the D600 is miles above the IQ of the D3/D700, but the reliability repoducability and dependability of the D600 is not perfect. I got used to it and produced great results. The RAW files have a depth and breadth of editability. WOW. I also used the viewfinder even for manual focus with difficult lenses but it is not easy. Errgonomics is good even without a grip.
of the newer cameras the D500 is a gem, I am even undecided if I like the D500 better or the D850.
Note that the D500 is half frame but runs circles around the D3 in any department even IQ. If had to choose between a used D500, D600, D700 I would sure buy the D500. Absolutely stunning professional hardware.