I haven't listened much to country and bluegrass but I used to sing in a band that did traditional & contemporary songs of Ireland, Scotland, and England. We were especially known for three-part harmony. When we sang three-part we wanted all levels equal so that the melody didn't stick out over the harmony and our hamonies didn't follow standard conventions, that is, you often couldn't tell where one part left off and another started. It make picking out the melody harder but the result was a sound that featured not just a lead singer but three singers singing together equally.