John Gill Commentary Deuteronomy 3:20

"until Jehovah give rest unto your brethren, as unto you, and they also possess the land which Jehovah your God giveth them beyond the Jordan: then shall ye return every man unto his possession, which I have given you." — Deuteronomy 3:20 (ASV)

Until the Lord have given rest to your brethren, as well as
to you
Rest from their enemies, and habitations to dwell quietly in; so the land of Canaan is called a rest, (Deuteronomy 12:9) typical of the rest which remains for the people of God:

and until they also possess the land which the Lord your God has given
them beyond Jordan ;
for so Canaan was with respect to Moses and the people with him, who were then in the plains of Moab; otherwise the country in which he was with respect to Canaan is usually called beyond Jordan;

this the Lord had given in promise to Israel, and they were just now ready to enter into and possess it, by virtue of his gift, and which made it sure to them:

and then shall you return every man to his possession, which I have
given you ;
as accordingly they did, (Joshua 22:1–34) .