'three-leaf' originates from Modern English as a compound of the numeral 'three' and 'leaf' (Old English 'leaf'), where 'three' meant the number 3 and 'leaf' meant 'leaf' or 'leaflet'.

'three' comes from Old English 'þr
3e' (or 'þr
3'), and 'leaf' comes from Old English 'laf'; compounds meaning 'having N leaves' have been formed in English since at least Middle English, evolving into modern hyphenated forms like 'three-leaf'.

Initially used in a strictly literal sense to describe a plant part ('having three leaves'); it has remained largely literal, though it appears in set phrases like 'three-leaf clover'.