Deuteronomy 19:1-3

When the LORD thy God hath cut off the nations, whose land the LORD thy God giveth thee, and thou succeedest them, and dwellest in their cities, and in their houses; thou shalt separate three cities for thee in the midst of thy land, which the LORD thy God giveth thee to possess it. Thou shalt prepare thee a way, and divide the coasts of thy land, which the LORD thy God giveth thee to inherit, into three parts, that every slayer may flee thither.