Jamieson, Fausset and Brown on Joshua 18:4-9

4-9. Give out from among you three men for each tribe--Though the lot determined the part of the country where each tribe was to be located, it could not determine the extent of territory which might be required; and the dissatisfaction of the children of Joseph with the alleged smallness of their possession gave reason to fear that complaints might arise from other quarters, unless precautions were taken to make a proper distribution of the land. For this purpose a commission was given to twenty-one persons--three chosen from each of the seven tribes which had not yet received their inheritance, to make an accurate survey of the country.