trefoil (n.)

late 14c., type of clover, from Anglo-French trifoil (13c.), Old French trefueil "clover, clover-leaf," from Latin trifolium "three-leaved plant," from tri- "three" (see tri-) + folium "leaf" (from PIE root *bhel- (3) "to thrive, bloom"). As a type of ornamental figure in medieval architecture, early 15c.