The synonym for the word "third" is "tertian". Some other synonyms for "third" are "triennial", which refers to something that occurs every three years, and "trimestral", which means something that happens every three months. "Threefold" or "triple" also refer to the number three, and can be used in place of "third". Another synonym for "third" is "treble", which can be used to refer to a high-pitched musical note that is three times higher than another note. "Tierce" is another synonym that is used to describe the third item in a sequence or a third part in a composition.