𐬚𐬭𐬌 • (θri-)
García, Javier Martínez; de Vaan, Michiel (2014) Introduction to Avestan (Brill Introductions to Indo-European Languages; 1), Brill, ISBN 9789004258099, page 66
From Proto-Iranian [Term?], from Proto-Indo-Iranian [Term?] (compare Sanskrit त्रि (tri)), from Proto-Indo-European *tréyes.