Deuteronomy 19:1-3

When Jehovah thy God shall cut off the nations, whose land Jehovah thy God giveth thee, and thou succeedest them, and dwellest in their cities, and in their houses; thou shalt set apart three cities for thee in the midst of thy land, which Jehovah thy God giveth thee to possess it. Thou shalt prepare thee the way, and divide the borders of thy land, which Jehovah thy God causeth thee to inherit, into three parts, that every manslayer may flee thither.