A set of three is a common concept in many different fields. The term 'TRIO' is used to refer to a group of three musicians, while 'TRIPLET' is used to refer to three siblings born at the same time. A 'TRILOGY' is a set of three related works, such as books or movies. A 'TRIAD' is a group of three people or things that are closely related. A 'trifecta' is a set of three successes or achievements. A 'TERN' is a type of bird that typically lives in groups of three. Finally, a 'TRINE' is an astrological aspect between three planets.