noun

• (art) A picture or series of pictures painted on three tablets connected by hinges.
  "I never thought breastfeeding would be hard. When I thought about it at all, my mind conjured beatific scenes suffused with a sort of religious glow. […] These days I envisage more of a triptych: the infant Jesus spluttering at the breast, face purple with hangry fury; the infant Jesus possetting milk down Mary’s front; the infant Jesus and the nappy explosion."
• (by extension) A group of three people or works, especially when considered representative of a particular field or theme.
  "Thus, with the passing years, Papa Wemba, a hardworking and introverted man, has come to embody “African pop,” forming a triptych with Youssou N’Dour and Salif Keita."
• (by extension) A group of three people or works, especially when considered representative of a particular field or theme.
  "The centerpiece of the show is a poised, luminous, tender, and diabolically clever video installation, “Le Triptyque de Noirmoutier” (The Noirmoutier Triptych), which Varda shot on 35-mm. film[…]. The man gets himself a bottle of beer; the older woman heads with dishes from the sink toward the cupboard—and crosses the dividing line of the triptych exactly like crossing a threshold."
• (by extension) A group of three people or works, especially when considered representative of a particular field or theme.