John Gill Commentary Deuteronomy 3:16

"And unto the Reubenites and unto the Gadites I gave from Gilead even unto the valley of the Arnon, the middle of the valley, and the border [thereof], even unto the river Jabbok, which is the border of the children of Ammon;" — Deuteronomy 3:16 (ASV)

And unto the Reubenites, and unto the Gadites
The tribes of Reuben and Gad:

I gave from Gilead even unto the river Arnon :
see (Deuteronomy 3:12)

half the valley and the border ;
or rather half the river, the river Arnon; and so it is rendered "the middle of the river", in (Joshua 12:2) and so here the middle of the torrent by the Vulgate Latin and Septuagint versions, and by Onkelos:

even unto the river Jabbok, which is the border of the children of
Ammon ;
beyond which the land given to the tribes of Reuben and Gad reached not; see (Deuteronomy 2:37) .