I found this from...somewhere:

I converted a 21 chord Schmidt into a three key, G D A harp.

Mine has 12 chords, the row nearest the high strings E, D7, A7, E7, ; middle row C, G, D, A,; row toward low strings Am, Em, Bm, F#m. There might be a better choice than the E, but I have songs I need it for, so I'm happy with it.

I would suggest that you keep your key configuration the same for whatever
harps you use (so you reach your 7ths and minor chords the same way) though,
of course I've broken this rule for the E.




I play a 3 key diatonic (G,D,A) mostly, but also own an F/C.


Saul