I've tried it on two computers -- both my nMP and my rMB, and when I install iTunes 12.3 I can no longer tether my iPhone over USB. Under Network Preferences on both Macs the iPhone USB entry simply says "Not Connected." I know this happened back with the original iTunes 11 release and the fix back then was to replace the Ethernet-over-USB kext with an older version. Sadly, that fix doesn't work in this case. Ah, Apple giveth and Apple taketh away. I guess I'll use wi-fi tethering until this gets fixed.