An Etymological Dictionary of the German Language/Annotated/drei

by Friedrich Kluge

drei, numeral, ‘three,’ from Middle High German and Old High German drî, which is properly simply the nominative masculine; the rest of the old cases are obsolete in Modern High German; Anglo-Saxon þrî, þreó, English three, Gothic þreis, from *þrijis. It corresponds to Aryan trejes, equivalent to Sanscrit tráyas, Greek τρεῖς, from τρέγες, Latin três, Old Slovenian trĭje. Drei, like the other units, is a primitively word. See Drillich, Dritte.