Thou shalt separate three cities - See on Numbers 35:11 (note), etc.

Thou shalt separate three cities for thee in the midst of thy 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 but these were to be in the midst of the land of Canaan; see Joshua 20:7,
which the Lord thy God giveth thee 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.

OF THE CITIES OF REFUGE. (Deuteronomy 19:1-13)
Thou shalt separate three cities for thee in the midst of thy land--Goelism, or the duty of the nearest kinsmen to avenge the death of a slaughtered relative, being the customary law of that age (as it still is among the Arabs and other people of the East), Moses incorporated it in an improved form with his legislative code. For the protection of the unintentional homicide, he provided certain cities of refuge--three had been destined for this purpose on the east of Jordan (Deuteronomy 4:41; Numbers 35:11); three were to be invested with the same privilege on the west of that river when Canaan should be conquered.
in the midst of thy land--in such a position that they would be conspicuous and accessible, and equidistant from the extremities of the land and from each other.

In the midst of the land - Namely, beyond Jordan, as there were three already appointed on this side Jordan: In the midst of the several parts of their land, to which they might speedily flee from all the parts of the land.