The person you likely recognize is Leonard Rumery. In an article with The News-Gazette (Champaign, IL) he explains he was credited as First Farmhand.   Based on that, the other members of the band are probably William M. Paris (Second Farmhand) and Kathleen Rainey (Third Farmhand).

Here's a clip of the latter appearance (link). I also managed to find a fully copy of the movie online on the Internet Archive, and believe the group you're referring to first appears about 22 minutes in and more importantly about 1 hour 19 minutes in. As it turns out, he introduced them by their real names: Cathy, Leonard, and Bill.