Where does “trithing” come from?

trithing (English) comes from Middle English trithing, from Old English þriðing, from Old Norse þriðjungr, from Proto-Germanic þridjô, from Proto-Indo-European tretyós, from Proto-Indo-European tr̥tyós, from Proto-Indo-European tréyes — three.

trithing (English): One of three ancient divisions of a county in...