John Gill Commentary Numbers 3:42

"and Moses numbered, as Jehovah commanded him, all the first-born among the children of Israel." — Numbers 3:42 (ASV)

And Moses numbered, as the Lord commanded him
No doubt assisted by others, though not mentioned:

all the firstborn among the children of Israel ;
which some think were only those that were born since they came out of Egypt, as Bonfrerius, Bishop Patrick, and others.