Albert Barnes 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)

The sense is that the Reubenites and Gadites were to possess the district from the Jabbok on the north to the Arnon on the south, including the middle part of the valley of the Arnon, and the territory (“coast” or “border”) pertaining to it.