Num 3:2-4 In order to indicate at the very outset the position which the Levites were to occupy in relation to the priests (viz., Aaron and his descendants), the account of their muster commences not only with the enumeration of the sons of Aaron who were chosen as priests (Num 3:2-4), but with the heading: “These are the generations of Aaron and Moses in the day (i.e., at the time) when Jehovah spake with Moses in Mount Sinai (Num 3:1). The toledoth (see at Gen 2:4) of Moses and Aaron are not only the families which sprang from Aaron and Moses, but the Levitical families generally, which were named after Aaron and Moses, because they were both of them raised into the position of heads or spiritual fathers of the whole tribe, namely, at the time when God spoke to Moses upon Sinai. Understood in this way, the notice as to the time is neither a superfluous repetition, nor introduced with reference to the subsequent numbering of the people in the steppes of Moab (Num 26:57.). Aaron is placed before Moses here (see at Exo 6:26.), not merely as being the elder of the two, but because his sons received the priesthood, whilst the sons of Moses, on the contrary, were classed among the rest of the Levitical families (cf. 1Ch 23:14).
Names of the sons of Aaron, the “anointed priests (see Lev 8:12), whose hand they filled to be priests,” i.e., who were appointed to the priesthood (see at Lev 7:37). On Nadab and Abihu, see Lev 10:1-2. As they had neither of them any children when they were put to death, Eleazar and Ithamar were the only priests “in the sight of Aaron their father,” i.e., during his lifetime. “In the sight of:” as in Gen 11:28.
The Levites are placed before Aaron the priest, to be his servants.