Where does “τρῐ́βρᾰχῠς” come from?

τρῐ́βρᾰχῠς (Ancient Greek) comes from Ancient Greek τρῐ-, from Ancient Greek τρεῖς, from Proto-Hellenic tréyes, from Proto-Indo-European *tréyes.

τρῐ́βρᾰχῠς (Ancient Greek): tribrach metrical foot comprising three short syllables