Trio sonata
A trio sonata is written for three voices (three parts), in other words, it can be played by three different instruments.  The two upper voices share the same musical themes.  They often imitate one another.  They are of equal importance.  The third part, the lowest part, is played by basso continuo, which means the accompaniment.  This continuo is usually played by two instruments:  typically a cello or bass viol and a keyboard instrument such as the harpsichord.  That makes four players, of course.  However, the harpsichord player’s left hand notes are the same as the cello’s notes.  This is the “third” part of the trio.