Peter, Paul, and Mary were an American folk music TRIO formed in 1961. They are best known for their hit singles 'If I Had a Hammer' and 'Blowin' in the Wind'. The TRIO consisted of Peter Yarrow, Paul Stookey, and Mary Travers. They were one of the most successful folk-singing groups of the 1960s and their music has been influential to many other artists. The crossword solution for the term Peter, Paul, and Mary, for example is TRIO, which is a group of three people.