Ok. I was told the usb 3 connector has two banks for the reason that one of them could take an usb 2 cable - but I had not checked which usb 2 it would have to be, or if it only works with a different usb 3 terminal (not the micro).
According to teathertools:
The D8x0 and D5 use USB 3 and share the same connector - Micro B.
The Df, D7200, D750 use USB 2 with Mini B 8 pin connector.
The D3400 uses USB 2 with Micro B 5 pin connector.
The D100/D200/D300/D600/D700/D1/D2/D3/D4 use USB 2 with Mini B 5-pin connector.
So in practice you just need a bunch of cables around to connect various cameras to computer via USB.