Where does “τρίγλυφος” come from?

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

τρίγλυφος (Ancient Greek): thrice-cloven; triglyph, a three-grooved tablet...