Where does “третии” come from?

третии (Old Church Slavonic) comes from Proto-Slavic tretь, from Proto-Balto-Slavic tretias, from Proto-Balto-Slavic tirtias, from Proto-Indo-European tr̥tyos, from Proto-Indo-European *tréyes.

Ancestry of “третии”, step by step

StepLanguageWordMeaning
1Proto-Slavictretьthird
2Proto-Balto-Slavictretias—
3Proto-Balto-Slavictirtias—
4Proto-Indo-Europeantr̥tyos—
5Proto-Indo-European*tréyes—

Words derived from “третии”

Every word from Proto-Indo-European *tréyes →Every word from Proto-Indo-European tr̥tyos →Every word from Proto-Balto-Slavic tirtias →