Where does “три” come from?

три (Old East Slavic) comes from Proto-Slavic trьje, from Proto-Balto-Slavic tríjes, from Proto-Indo-European *tréyes.

Ancestry of “три”, step by step

StepLanguageWordMeaning
1Proto-Slavictrьjethree
2Proto-Balto-Slavictríjesthree
3Proto-Indo-European*tréyes—
Every word from Proto-Indo-European *tréyes →Every word from Proto-Balto-Slavic tríjes →Every word from Proto-Slavic trьje →