The compound noun "three-block" is a transparent combination of the numeral "three," with ancient Indo-European roots, and the noun "block," of Germanic origin via French and Dutch. In its specialized nautical sense, "block" refers to a pulley or a specific unit of length related to a ship's equipment. The term literally denotes a measure equaling three such units. Its logic stems from practical seamanship, where distances for signaling, anchoring, or maneuvering were calculated based on multiples of this fundamental "block" length, evolving into a standardized technical term.

Structure: three (number) + block (unit) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a numerical quantifier. - block: From Middle English blok, from Old French bloc, from Middle Dutch blok. Functions as a noun denoting a solid piece or a unit of measurement.