Musar על יהושע 3:3

The Parshah mentions only the number of Israelites, and the fact that the Levites from the age of thirty days and over were chosen to replace the firstborn. The number given for the Levites in this Parshah is only that of the Kehatites, since the מחנה שכינה contained the כהנים who came from their ranks. The reason why the כהנים were included in the count of the Kehatites is that we find the Priests carrying the Holy Ark on three occasions, though that assignment was usually reserved for the Levites of the family of the Kehatites. These three occasions were: A) When Israel crossed the Jordan (Joshua 3,3), see the commentary of Kimchi there. B) When they marched around the city of Jericho (Joshua 6,12-13), and C) when the Holy Ark was returned to where it belonged during the uprising of Absalom against David (Samuel II 15,29).