The so-called ensemble of three musicians - instrumental or less vocal. Just called and works written for the actors. However, a piece for three singers often called teracom.
Most often composers write a trio for violin, cello and piano. Maybe you've heard of "in memory of the great artist", dedicated to the author - Tchaikovsky - the memory of Nikolai Rubinstein? This trio began the tradition of Russian music and that is what the work is dedicated to the memory of the departed. His "Elegiac trio, written in the year of death of Tchaikovsky, Rachmaninoff dedicated to his memory.
S. Rachmaninoff. Elegiac trio for violin, cello and piano. No. 2, Op. 9
And Shostakovich Second trio was dedicated to the memory of my friend, a prominent musical figure I. I. Sollertinsky.
Meet the trio, written for other instruments. For example, "Pathetic trio" Glinka is intended for clarinet, bassoon and piano.
There is the word trio another meaning. This is the middle section of a minuet or Scherzo. And here's why: once upon a time in the minuet the middle part only played three instruments. Then the minuet became a Symphony, Sonata, Quartet as one of the secondary parts. Still later instead minuet composers began to write symphonies Scherzo. And, traditionally, the middle section has remained the same definition.