Where does “ছত্তীছগঢ়” come from?

ছত্তীছগঢ় (Assamese) comes from Hindi छत्तीसगढ़, from Hindi छत्तीस, from Sanskrit षट्त्रिंशत्, from Sanskrit त्रिंशत्, from Proto-Indo-Aryan triHśát, from Proto-Indo-Iranian triHćát, from Proto-Indo-European tridḱómt, from Proto-Indo-European tri- — three.

Ancestry of “ছত্তীছগঢ়”, step by step

Step	Language	Word	Meaning
1	Hindi	छत्तीसगढ़	—
2	Hindi	छत्तीस	thirty-six
3	Sanskrit	षट्त्रिंशत्	thirty-six
4	Sanskrit	त्रिंशत्	thirty
5	Proto-Indo-Aryan	triHśát	—
6	Proto-Indo-Iranian	triHćát	thirty
7	Proto-Indo-European	tridḱómt	thirty
8	Proto-Indo-European	tri-	three, threefold
9	Proto-Indo-European	tréyes	three

Every word from Proto-Indo-European tréyes →Every word from Proto-Indo-European tri- →Every word from Proto-Indo-European tridḱómt →