I suggest that in major keys you dont double the third note.in tonic solfa thats the me note of the chord. a chord is do me and soh ,ist 3rd 5th.try also to keep the bass do or so, as much as you can. when working out harmonies. with minor chords doubling the me note can be o k. your experimantation with A T B should be ok, I would keep away from concertina sites they are full of pedants and assholes.