Albert Barnes Commentary Joshua 18:4

"Appoint for you three men of each tribe: and I will send them, and they shall arise, and walk through the land, and describe it according to their inheritance; and they shall come unto me." — Joshua 18:4 (ASV)

Three men for each tribe - that is, 21 in all. Their duty would be to describe the land, especially with reference to the cities it contained (Joshua 18:9), that Joshua might have the means of making a first apportionment among the tribes according to their varying numbers.