“And no man has ascended into Heaven, but he who descended out of Heaven, even the Son of Man who is in Heaven.”
In Proverbs 30:4 the question is asked, “Who has ascended up into heaven and descended?” and the expected answer is ‘nobody'. For as Jesus brings out here, the only One Who can ascend into Heaven is One Who has first descended. Only such a one can ascend to control the ‘ruach' (Spirit, wind) and the rain (Proverbs 30:4). Thus the ‘ascending' refers to Jesus exercising His power over things above. Jesus will later stress that He cannot work without the Father being present with Him. But the writer may well also wish us to gain the hint of His final ascension.
What are the ‘heavenly things' of John 3:11? Firstly that Jesus has come from His glory in Heaven and has been made man. Secondly that He is the Son of man who has access to Heaven and Heaven's secrets (compare Matthew 11:25-27). And thirdly that He alone is able to enter Heaven (compare John 6:62) as the glorious Son of Man to receive the kingdom and the power and the glory (Daniel 7:13), for ‘no one has ascended up into heaven except He Who descended from heaven, even the Son of Man'.
Jesus has already declared Himself to be the Son of Man (John 1:51), and now He links the title with the heavenly Son of Man (Daniel 7:13), as He does also in the other Gospels. He Himself is the One Who has come down from heaven, and maintains contact with Heaven (compare John 1:51) and can therefore finally return to His heavenly home in triumph as the glorious Son of Man.
‘Who is in Heaven'. This is omitted in many manuscripts, and although it is fairly strongly evidenced the weight of evidence must be seen as against it (p66, Aleph, B, L, W omit it. A Theta f1 f13 include it). However, the idea behind it, that Jesus has access to Heaven's secrets is unquestionable (Matthew 11:25-27).