Deuteronomy 19:8-9 Three More Cities!

 8And if the LORD your God enlarges your territory, as He swore to your fathers, and gives you all the land He promised them, 9and if you carefully keep all these commandments I am giving you today, loving the LORD your God and walking in His ways at all times, then you are to add three more cities to these three.

As their territory expanded there needed to be three more cities of refuge added evenly dispersed or the avenger of blood would over take the manslayer.

Joshua 20:7-8 tells of the actual refuge cities chosen;

Joshua 20: 7So they set apart Kedesh in Galilee in the hill country of Naphtali, Shechem in the hill country of Ephraim, and Kiriath-arba (that is, Hebron) in the hill country of Judah.

8And beyond the Jordan, east of Jericho, they designated Bezer on the wilderness plateau from the tribe of Reuben, Ramoth in Gilead from the tribe of Gad, and Golan in Bashan from the tribe of Manasseh.

Accidents happen and unfortunately innocent people die because of them. The Levites would hear the case and investigate the death and make a determination of innocence or guilt.

Blessings