A set of three is often referred to as a trio or a triad. Other synonyms for this term include triplet, threesome, threesome set, and trifecta. If you're looking for a more literary or artistic phrase, you could use "tercet," "triptych," or "trilogy." A set of three can also be described as a triumvirate, if the three items or individuals have equal power or authority. When it comes to specific objects, a set of three may be called a trilogy (for books or movies), a triquetra (for a Celtic knot design), or a triptych (for a painting or photograph that's divided into three parts).