Do you see the little green MIDI indicator in the bottom right-hand corner of the Dorico project window flash when you play notes on your keyboard, LWestendorf?
I have the same problem. My Casio Privia PX-160 has played sometimes, but other times there is NOTHING I can do to get the keyboard to register notes in Dorico 3. When it has worked, the lower right corner showed the notes and the notes came up on Dorico. Now neither happens.
Check whether or not the keyboard shows up as a MIDI input device in the MIDI Input Devices dialog, accessed via the Play page of Preferences.