C3 not declared in this scope when running MIDI example code
The pitches for the notes are taken from pitchToNote.h. In there you can see definitions for the notes like:

#define pitchC3  48


It seems that the names were changed from C3 to pitchC3.

I am not 100% sure, but my guess is that this was done to avoid conflicts between the note A3 and the analog input A3 (and silimar).

So, in line 25 of the example code, add pitch for all notes:

const byte notePitches[NUM_BUTTONS] = {pitchC3, pitchD3, pitchE3, pitchF3, pitchG3, pitchA3, pitchB3};