Where does “третья” come from?
третья (Russian) comes from Russian тре́тий, from Old East Slavic треть, from Proto-Slavic tretь, from Proto-Balto-Slavic tretias, from Proto-Balto-Slavic tirtias, from Proto-Indo-European tr̥tyos, from Proto-Indo-European *tréyes.
третья (Russian): third
Ancestry of “третья”, step by step