Adam Clarke on John 3:13

No man hath ascended - This seems a figurative expression for, No man hath known the mysteries of the kingdom of God; as in Deu 30:12; Psa 73:17; Pro 30:4; Rom 11:34. And the expression is founded upon this generally received maxim: That to be perfectly acquainted with the concerns of a place, it is necessary for a person to be on the spot. But our Lord probably spoke to correct a false notion among the Jews, viz. that Moses had ascended to heaven, in order to get the law. It is not Moses who is to be heard now, but Jesus: Moses did not ascend to heaven; but the Son of man is come down from heaven to reveal the Divine will.

That came down - The incarnation of Christ is represented under the notion of his coming down from heaven, to dwell upon earth.

Which is in heaven - Lest a wrong meaning should be taken from the foregoing expression, and it should be imagined that, in order to manifest himself upon earth he must necessarily leave heaven; our blessed Lord qualifies it by adding, the Son of man who is in heaven; pointing out, by this, the ubiquity or omnipresence of his nature: a character essentially belonging to God; for no being can possibly exist in more places than one at a time, but He who fills the heavens and the earth.