'three-pitched' originates as a modern English compound of the numeral 'three' and the past-participle/adjectival form 'pitched' (from the verb 'pitch'), where 'three' ultimately comes from Old English 'þrīe/þrēo' meaning 'three' and 'pitch' here derives from the verb sense meaning 'to set a pitch (tone) or to give a slope/angle'.

'three' changed from Old English forms 'þrīe/þrēo' to Middle English and then modern English 'three'; 'pitch' developed multiple senses in Old and Middle English (relating to tone, to throw or set, and to slope), and combining the numeral with the past-participle produced the descriptive compound 'three-pitched' in modern usage.

Initially the elements meant simply '3' and 'pitched' (the state of being pitched); over time the compound has been used specifically to describe things that have 'three pitches' in the sense of either three musical tones or three sloping surfaces.