Complete Commentary on the Whole Bible, by Matthew Henry, at sacred-texts.com
(Num 3:1-13) The sons of Aaron, The Levites taken instead of the first-born.
(v. 14-39) The Levites numbered by their families, Their duties.
(Num 3:40-51) The first-born are numbered.
There was much work belonging to the priests' office, and there were now only Aaron and his two sons to do it; God appoints the Levites to attend them. Those whom God finds work for, he will find help for. The Levites were taken instead of the first-born. When He that made us, saves us, as the first-born of Israel were saved, we are laid under further obligations to serve him faithfully. God's right to us by redemption, confirms the right he has to us by creation.
The Levites were in three classes, according to the sons of Levi; Gershon, Kohath, and Merari; and these were subdivided into families. The posterity of Moses were not at all honoured or privileged, but stood upon the level with other Levites; thus it was plain, that Moses did not seek the advancement of his own family, or to secure any honours to it. The tribe of Levi was by much the least of all the tribes. God's chosen are but a little flock in comparison with the world.
The number of the first-born, and that of the Levites, came near to each other. Known unto God are all his works beforehand; there is an exact proportion between them, and so it will appear, when they are compared together. The small number of first-born, over and above the number of the Levites, were to be redeemed, and the redemption-money given to Aaron. The church is called the church of the first-born, which is redeemed, not as they were, with silver and gold; but, being devoted by sin to the justice of God, is ransomed with the precious blood of the Son of God. All men are the Lord's by creation, and all true Christians are his by redemption. Each should know his own post and duty; nor can any service required by such a Master be rightly accounted mean or hard.