An Etymological Dictionary of the German Language/drei

by Friedrich Kluge

drei, num., ‘three,’ from MidHG. and OHG. drî, which is prop. simply the nom. masc.; the rest of the old cases are obsolete in ModHG.; AS. þrî, þreó, E. three, Goth. þreis, from *þrijis. It corresponds to Aryan trejes, equiv. to Sans. tráyas, Gr. τρεῖς, from τρέγες, Lat. três, OSlov. trĭje. Drei, like the other units, is a primit. word. See Drillich, Dritte.