Commentary for Deuteronomy 19:2

שלוש ערים תבדיל, after Moses had explained the function of the judges, the king, the priests, and the prophets, Moses now details some of the tasks that fall under the jurisdiction of the judges, among them the establishment of the cities of refuge, the boundaries of the land, the examination of witnesses to establish that their testimony is truthful and accurate, and that they are acceptable as witnesses. Next, he deals with the tasks that fall within the competence of the king, such as leading the nation in war, especially aggressive wars beyond the boundaries of the land of Israel. (20, 1-9) The priests also have a role to play during the preparation of what are termed מלחמת רשות, “halachically permissible wars,” although they are not fought in order to repel invaders. The priests and judges cooperate in some of their duties, especially the עגלה הערופה, the calf killed when a murder in the field was unresolved. (21,1-9)

Separate three cities for yourself. Do not think that it would be better to place the cities of refuge in another kingdom, where the residents would be safer from the relatives of the deceased, who will not be able to cross the border without permission from the ruler of that land. This is not the case, but rather the cities must specifically be within your land.