Deuteronomy 19:1-3

When the LORD your God cuts off the nations, whose land the LORD your God gives you, and you succeed them, and dwell in their cities, and in their houses;

you shall set apart three cities for yourselves in the midst of your land, which the LORD your God gives you to possess it.

You shall prepare the way, and divide the borders of your land, which the LORD your God causes you to inherit, into three parts, that every man slayer may flee there.