Possible Clues:

• Dixie Chicks, e.g.
• The Magi, e.g
• The Powerpuff Girls, e.g
• Destiny's Child, e.g
• The Supremes, e.g
• Peter, Paul and Mary, e.g
• Group of three performers
• Wynken, Blynken and Nod, e.g
• Pair plus one
• Duo + one