וַיִּוָּלֵ֣ד לְאַהֲרֹ֔ן אֶת־נָדָ֖ב וְאֶת־אֲבִיה֑וּא אֶת־אֶלְעָזָ֖ר וְאֶת־אִיתָמָֽר׃
At ipinanganak kay Aaron sina Nadab at Abihu, sina Eleazar at Itamar.
Rabbeinu Bahya
Another approach: There is a mystical element here when the Torah writes of the four sons of Aaron, two of whom had already died and who certainly were not part of this census anymore. The Torah had described Nadav and Avihu as having died “before the Lord” (verse 4). It is reasonable then to understand the words “these are the sons of Aaron” as referring only to Nadav and Avihu whose death is being repeated in verse 4. Seeing that all four of Aaron’s sons had been anointed with sacred oil, we would have to understand the word לכהן at the end of verse 3, normally translated as: “to function as priest,” as meaning לכוהן, “to be priests,” a reference to someone who would function as priest after his father had died, i.e. Pinchas son of Eleazar. This would also account for Exodus 6,25: “she bore for him Pinchas the heads of the Levites according to their families.” This is also why again at the beginning of Parshat Pinchas (Numbers 26,60-61) the birth and death of Nadav and Avihu is once again mentioned. This is also why in Numbers 25,11 the Torah troubled itself to again trace Pinchas’ genealogy back to his grandfather Aaron. Seeing that the entire Torah is full of hints with profound meanings the Torah here saw fit to refer to these sons of Aaron three times. The first mention covers the intellectual relationship of these sons to their uncle Moses. The second time their existence is mentioned is a reference to their physical existence. The third time they are mentioned was to reveal to us something about the nature of G’d’s system of Justice, how instead of indulging the great it does the reverse and measures each one of their actions minutely. Seeing that two of these insights into G’d’s mind were revealed at the same time, the Torah used the conjunctive letter ו in introducing chapter three with the word ואלה. Seeing that the sons of Aaron had to be mentioned, the Torah also had to repeat what happened to them and why.