A.T. Robertson Commentary John 3:13

"And no one hath ascended into heaven, but he that descended out of heaven, [even] the Son of man, who is in heaven." — John 3:13 (ASV)

But he that descended out of heaven (ε μη ο εκ του ουρανου καταβας). The Incarnation of the Pre-existent Son of God who was in heaven before he came down and so knows what he is telling about "the heavenly things." There is no allusion to the Ascension which came later. This high conception of Christ runs all through the Gospel and is often in Christ's own words as here.

Which is in heaven (ο ων εν τω ουρανω). This phrase is added by some manuscripts, not by Aleph B L W 33, and, if genuine, would merely emphasize the timeless existence of God's Son who is in heaven even while on earth. Probably a gloss. But "the Son of man" is genuine. He is the one who has come down out of heaven.