klaasvakie I like the restriction on the melody but not on the harmony. For the harmony, I'd say all incidentals are allowed. That opens up a few more chords.
Or even just make it 4 notes. That opens up a lot of options. But I feel like it might scare beginners whereas just limiting the melody (while leaving the harmony open) means you can pick 3 notes and go bos ? Open to anyone of any skill level.