The compound adjective 'three-digit' is a transparent modern English formation. Its first element, 'three', descends from the Proto-Indo-European root trei-, which gave rise to the concept of the number across many Indo-European languages, including Latin tres and Greek treis. The second element, 'digit', has a more figurative journey from the Latin digitus for 'finger' or 'toe', reflecting the ancient practice of counting on one's fingers. By the late Middle English period, 'digit' had acquired its specific mathematical sense for any of the numerals from 0 to 9. Thus, 'three-digit' logically combines the ancient term for the quantity with the body-part-turned-numeral to precisely denote anything composed of three such numerical symbols.

Structure: three (number) + digit (numeral) - three: From Old English þrīe, þrēo, of Germanic origin; related to Dutch drie and German drei, from an Indo-European root shared by Latin tres and Greek treis. Functions as the numerical quantifier. - digit: From Latin digitus meaning 'finger, toe'. In English, it came to mean a numeral from 0 to 9 because counting was originally done on the fingers. Functions as the counted entity.