As a proper noun triplex
is (british) a safety glass for car windows, consisting of three layers (glass and mica).
As a noun triplet is
a group of three.
Having three parts; triple or threefold.
(architecture) Having three floors or other divisions.
A building with three apartments or divisions
(juggling) throwing motion where three balls are thrown with one hand at the same time.
A group of three.
One of a group of three.
One of a group of three siblings born at the same time to the same mother.
(music) A group of three notes played or written where two notes would ordinarily be; a form of tuplet.
(physics) A triquark.