New Defender's Study Bible Notes
3:2 sons of Aaron. Aaron’s sons were destined to succeed him in the priesthood. However, the two oldest, Nadab and Abihu, forfeited this privilege and even their lives by offering “strange fire,” unauthorized by God. Note Numbers 3:4 and refer to Leviticus 10:1-7.
3:15 Number the children of Levi. God’s previous command to take a census had specifically precluded the Levites (Numbers 1:49). Furthermore, the census of Levites was to include all males over a month old, not just men able to fight.
3:17 sons of Levi. The three tribes of Levites were the Gershonites, Kohathites and Merarites, positioned on the west side, south side, and north side of the tabernacle, respectively (Numbers 3:23,29,35).
3:27 the Amramites. One of the sons of Kohath was Amram, who in turn had been the father of Moses and Aaron (Exodus 6:20). Since the descendants of Aaron had been anointed as priests, and were in charge of all the Levites, the privilege of the eastern side of the tabernacle (and leading the Levites in marching) was reserved for Moses, Aaron and his sons (Numbers 3:38). Aaron’s son Eleazar was appointed “chief over the chief of the Levites” (Numbers 3:32).
3:43 the firstborn males. The firstborn males in all the families of Israel (not including any fathers, of course) totaled 273 more than all the Levites (compare Numbers 3:39 and 43). God’s provision had been that each firstborn son would belong to the Lord, but that he would be redeemed, as it were, by a Levite–who in turn must be supported (implicitly) by that family as its representative Levite who would devote his life to God in the tabernacle (Numbers 3:41). Since no Levite were available for the extra 273 firstborn sons, the latter were to be redeemed by payment of five shekels each.