The compound "three-thirty" is a modern English construction formed by directly combining two cardinal numbers. The first element, "three," descends from the Proto-Indo-European root trei-, which gave rise to the number across many Indo-European languages, signifying a quantity of one more than two. The second element, "thirty," is itself a compound from Old English, literally meaning "three tens" or "three times ten." When combined, the phrase follows the standard English convention for telling time, where the hour ("three") is linked with the minutes ("thirty") to denote a specific point 30 minutes past that hour. This logical, additive structure makes the meaning of the time immediately transparent.

Structure: three (numeral) + thirty (numeral) - three: From Old English þrīe, þrēo, of Proto-Germanic origin; related to Dutch drie and German drei, from an Indo-European root shared by Latin tres and Greek treis. Functions as the cardinal number 3. - thirty: From Old English þrītiġ, from þrīe (three) + -tiġ (group of ten). Functions as the cardinal number 30.