Thanks Glen!
Now back to a more basic question:
I am not sure if I am using the right definition of three part or four part harmony. Three or four people singing together on the same scale but one singing lead (fundamental) one singing on the fourth scale note and one on the fifth scale note to form a triad (chord of the fundamental key) and I believe this is what happens with barbershop with the bass an octave below the lead for four part.
Another type of harmony would be a tenor, a baritone and a bass singing on the same note but an octave apart and this is what I hear more often with Bluegrass. When bluegrass notes blend in a triad the music often seems to stop or drop. I wonder if my ear is just confused by the music?