The phrase denotes a Halloween costume designed for a group of three. It implies coordinated outfits and a cohesive theme shared among the individuals. For example, three people might dress as the characters from a well-known fairy tale or as related objects in a humorous set.
The appeal lies in its collaborative nature and enhanced visual impact. Such coordinated ensembles allow for more elaborate and creative representations than single costumes. Historically, group costuming has been a way to showcase shared identity, friendship, or participation in communal events. This approach to Halloween attire leverages this tradition, emphasizing unity and coordinated presentation for maximum effect.