Where does “τριήμερος” come from?

τριήμερος (Greek) comes from Greek ημέρα, from Ancient Greek Ἡμέρα, from Ancient Greek ἦμαρ, from Proto-Hellenic ā́mər, from Proto-Indo-European h₂eh₃mr̥, from Proto-Indo-European h₂eh₃- — to be hot; to burn.

τριήμερος (Greek): triduan, three-day lasting three days