Peter, Paul and Mary, e.g.

TRIO

Think of a small group often seen performing together, like a musical act with exactly three members.

This clue refers to the folk music group Peter, Paul and Mary, which famously had three members. The phrase "e.g." signals that they’re an example of the answer, which is a word for a group of three. "Trio" fits perfectly since it means exactly that—a set of three people, often in music or performance.