'three-coloured' originates from Modern English, specifically the compound of 'three' and 'coloured', where 'three' meant 'the number 3' and 'coloured' derived from 'colour' meaning 'hue'.

'coloured' comes from Middle English 'colour'/'colouren', from Old French 'colour', from Latin 'color, coloris'; 'three' comes from Old English 'þrīe' (from Proto-Germanic *þrīz). The compound 'three-coloured' was formed in Modern English by combining these elements.

Initially it meant 'having three colours' and this literal meaning has largely remained unchanged in modern usage.