John Gill Commentary Deuteronomy 19:2

"thou shalt set apart three cities for thee in the midst of thy land, which Jehovah thy God giveth thee to possess it." — Deuteronomy 19:2 (ASV)

You shall separate three cities for you in the midst of your land From the cities they took possession of and dwelt in; and indeed from the cities of the Levites, which were given to them to inhabit; three were before ordered to be separated from those inhabited by the tribes of Reuben and Gad, and the half tribe of Manasseh, (Deuteronomy 4:41–43) but these were to be in the midst of the land of Canaan; see (Joshua 20:7Joshua 20:8) ,

which the Lord your God gives you to possess it : which as it is often mentioned when this land is spoken of, so it carries in it a reason here why this order of the Lord's should be readily complied with, the whole land and all the cities of it being the gift of his to them.