A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
num 3:1THE LEVITES' SERVICE. (Num. 3:1-51)
These . . . are the generations of Aaron and Moses, &c.--This chapter contains an account of their families; and although that of Moses is not detailed like his brother's, his children are included under the general designation of the Amramites (Num 3:27), a term which comprehends all the descendants of their common father Amram. The reason why the family of Moses was so undistinguished in this record is that they were in the private ranks of the Levites, the dignity of the priesthood being conferred exclusively on the posterity of Aaron; and hence, as the sacerdotal order is the subject of this chapter, Aaron, contrary to the usual style of the sacred history, is mentioned before Moses.
in the day that the Lord spake with Moses in mount Sinai--This is added, because at the date of the following record the family of Aaron was unbroken.
num 3:2And these are the names of the sons of Aaron--All the sons of Aaron, four in number, were consecrated to minister in the priest's office. The two oldest enjoyed but a brief term of office (Lev 10:1-2; Num 3:4; Num 26:61); but Eleazar and Ithamar, the other two, were dutiful, and performed the sacred service during the lifetime of their father, as his assistants, and under his superintendence.
num 3:5Bring the tribe of Levi near--The Hebrew word "bring near" is a sacrificial term, denoting the presentation of an offering to God; and the use of the word, therefore, in connection with the Levites, signifies that they were devoted as an offering to the sanctuary, no longer to be employed in any common offices. They were subordinate to the priests, who alone enjoyed the privilege of entering the holy place; but they were employed in discharging many of the humbler duties which belonged to the sanctuary, as well as in various offices of great utility and importance to the religion and morals of the people.
num 3:9they are wholly given unto him out of the children of Israel, &c.--The priests hold the place of God, and the Levites are the servants of God in the obedience they render to the priests.
num 3:11I have taken the Levites, &c.--The consecration of this tribe did not originate in the legislative wisdom of Moses, but in the special appointment of God, who chose them as substitutes for the first-born. By an appointment made in memory of the last solemn judgment on Egypt (from which the Israelitish households were miraculously exempt) all the first-born were consecrated to God (Exo 13:12; Exo 22:29), who thus, under peculiar circumstances, seemed to adopt the patriarchal usage of appointing the oldest to act as the priest of the family. But the privilege of redemption that was allowed the first-born opened the way for a change; and accordingly, on the full organization of the Mosaic economy, the administration of sacred things formerly committed to the first-born was transferred from them to the Levites, who received that honor partly as a tribute to Moses and Aaron, partly because this tribe had distinguished themselves by their zeal in the affair of the golden calf (Exo 32:29), and also because, being the smallest of the tribes, they could ill find suitable employment and support in the work. (See on Deu 33:8). The designation of a special class for the sacred offices of religion was a wise arrangement; for, on their settlement in Canaan, the people would be so occupied that they might not be at leisure to wait on the service of the sanctuary, and sacred things might, from various causes, fall into neglect. But the appointment of an entire tribe to the divine service ensured the regular performance of the rites of religion. The subsequent portion of the chapter relates to the formal substitution of this tribe.
I am the Lord--that is, I decree it to be so; and being possessed of sovereign authority, I expect full obedience.
num 3:14Number the children of Levi--They were numbered as well as the other tribes; but the enumeration was made on a different principle--for while in the other tribes the number of males was calculated from twenty years and upward [Num 1:3], in that of Levi they were counted "from a month old and upward." The reason for the distinction is obvious. In the other tribes the survey was made for purposes of war [Num 1:3], from which the Levites were totally exempt. But the Levites were appointed to a work on which they entered as soon as they were capable of instruction. They are mentioned under the names of Gershon, Kohath, and Merari, sons of Levi, and chiefs or ancestral heads of three subdivisions into which this tribe was distributed. Their duties were to assist in the conveyance of the tabernacle when the people were removing the various encampments, and to form its guard while stationary--the Gershonites being stationed on the west, the Kohathites on the south, and the families of Merari on the north. The Kohathites had the principal place about the tabernacle, and charge of the most precious and sacred things--a distinction with which they were honored, probably, because the Aaronic family belonged to this division of the Levitical tribe. The Gershonites, being the oldest, had the next honorable post assigned them, while the burden of the drudgery was thrown on the division of Merari.
num 3:32chief--rather, "chiefs" of the Levites. Three persons are mentioned as chiefs of these respective divisions [Num 3:24, Num 3:30, Num 3:35]. And Eleazar presided over them; whence he is called "the second priest" (Kg2 25:18); and in the case of the high priest's absence from illness or other necessary occasions, he performed the duties (Kg1 4:4).
num 3:38those that encamp, &c.--That being the entrance side, it was the post of honor, and consequently reserved to Moses and the priestly family. But the sons of Moses had no station here.
num 3:39twenty and two thousand--The result of this census, though made on conditions most advantageous to Levi, proved it to be by far the smallest in Israel. The separate numbers stated in Num 3:22, Num 3:28, Num 3:34, when added together, amount to twenty-two thousand three hundred. The omission of the three hundred is variously accounted for--by some, because they might be first-born who were already devoted to God and could not be counted as substitutes; and by others, because in Scripture style, the sum is reckoned in round numbers. The most probable conjecture is, that as Hebrew letters are employed for figures, one letter was, in the course of transcription, taken for another of like form but smaller value.
num 3:40Number all the first-born of the males of the children of Israel, &c.--The principle on which the enumeration of the Levites had been made was now to be applied to the other tribes. The number of their male children, from a month old and upward, was to be reckoned, in order that a comparison might be instituted with that of the Levites, for the formal adoption of the latter as substitutes for the first-born. The Levites, amounting to twenty-two thousand, were given in exchange for an equal number of the first-born from the other tribes, leaving an excess of two hundred seventy-three; and as there were no substitutes for these, they were redeemed at the rate of five shekels for each (Num 18:15-16). Every Israelite would naturally wish that his son might be redeemed by a Levite without the payment of this tax, and yet some would have to incur the expense, for there were not Levites enough to make an equal exchange. Jewish writers say the matter was determined by lot, in this manner: Moses put into an urn twenty-two thousand pieces of parchment, on each of which he wrote "a son of Levi," and two hundred seventy-three more, containing the words, "five shekels." These being shaken, he ordered each of the first-born to put in his hand and take out a slip. If it contained the first inscription, the boy was redeemed by a Levite; if the latter, the parent had to pay. The ransom-money, which, reckoning the shekel at half a crown, would amount to 12s. 6d. each, was appropriated to the use of the sanctuary. The excess of the general over the Levitical first-born is so small, that the only way of accounting for it is, by supposing those first-born only were counted as were males remaining in their parents' household, or that those first-born only were numbered which had been born since the departure from Egypt, when God claimed all the first-born as his special property.
num 3:41the cattle of the Levites--These, which they kept to graze on the glebes and meadows in the suburbs of their cities, to supply their families with dairy produce and animal food, were also taken as an equivalent for all the firstlings of the cattle which the Israelites at that time possessed. In consequence of this exchange the firstlings were not brought then, as afterwards, to the altar and the priests.