Trio : A traditional unit of quantity equal to 3. The word is Italian. It originated in music during the Baroque era (early 1700's) in its meaning as a composition in three parts
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}