John Gill Commentary Deuteronomy 3:8

"And we took the land at that time out of the hand of the two kings of the Amorites that were beyond the Jordan, from the valley of the Arnon unto mount Hermon;" — Deuteronomy 3:8 (ASV)

And we took at that time out of the hands of the two kings of
the Amorites
Sihon king of Heshbon, and Og king of Bashan;

the land that [was] on this side Jordan ;
where Moses then was, being in the plains of Moab, and was the country beyond Jordan, with respect to the land of Canaan, and when in that:

from the river of Arnon unto Mount Hermon ;
Arnon was a river which divided Moab and the Amorites, (Numbers 22:13) and Hermon was a mountain of Gilead, which ended where Lebanon began, and was the northerly border of this country. It was remarkable for the dew that fell on it; (See Gill on Psalms 133:3).