Deuteronomy 19:1-3, 9

“When the LORD your God cuts off the nations, whose land the LORD your God gives you, and you dispossess them and settle in their cities and in their houses, you shall set aside three cities for yourself in the midst of your land, which the LORD your God gives you to possess. You shall prepare the roads for yourself, and divide into three parts the territory of your land which the LORD your God will give you as a possession, so that any manslayer may flee there.

if you carefully observe all this commandment which I command you today, to love the LORD your God, and to walk in His ways always—then you shall add three more cities for yourself, besides these three.