Here's a few comments by Newman & Nida from the Translators handbook of John:
John 3:13
The purpose of this verse is to emphasize the heavenly origin of the Son of Man. John is the only one of the Gospel writers to emphasize this truth; it is basic to his theology. What gives the Son of Man his authority is his heavenly origin. The Son of Man … came down from heaven to tell men on earth about the things of heaven (verse 12). That is, the coming of the Son of Man is an act of divine revelation. But more than revelation is involved, as can be seen from the following verses—it is also an act of self-giving which leads to the death of the Son of Man.
Some scholars maintain that the verb has gone up refers to the Son of Man, and so implies that he had already ascended to heaven at the time these words were written. That is, they assume that this verse contains John’s comments about the Son of Man and that it reflects the post-resurrection theology of John, rather than the words of Jesus. It is thus one way of explaining the use of the perfect tense (has gone up).
However, the statement no one has ever gone up to heaven is possibly intended merely to deny that up to that time anyone had gone up to heaven to learn about the things of heaven. If this is the meaning, no reference to the ascension of the Son of Man is intended. Moreover, just as the first half of the verse denies that anyone else knows about the things of heaven, so the second half affirms that only the Son of Man has knowledge about those things, because his origin is in heaven. It is possible that the verse is intended to carry this meaning. However, in John’s Gospel the verb tenses are not always those one would expect, and it may be that the perfect tense here does not imply that the Son of Man has already ascended to heaven. For example, in 4:38 the past tense is used (TE V have sent) of an action which Jesus has not yet done, according to the time sequence of the Gospel. For translation it is suggested that the equivalent of a perfect tense (or even of a simple past tense) be used for has gone up, without the implication of “has gone up and is now there.”
Verse 13 involves a number of subtle problems for the translator. The use of the perfect tense “has gone up” would almost inevitably indicate this was a reference to the resurrection, and therefore would tend to be interpreted as a prophecy or an interpolation by the Gospel writer. To avoid such an implication the phrase except the Son of Man can be translated “except for the Son of Man, who will go up to heaven.” However, the focus of verse 13 is not on a future resurrection, but on the fact that the Son of Man has come down from heaven and is therefore in a position to reveal truth about God and his purposes for man’s salvation. In order to indicate this contrast vividly and in order to avoid the implication of an anachronism, it is possible to translate “but the Son of Man has come down from heaven.” Such a rendering avoids one difficulty by eliminating a possible interpretation. It seems best, however, simply to translate this verse, along with certain of its exegetical obscurities and ambiguities, and to leave the interpretation to commentators. Even though this solution might allow some slight confusion for the average reader, there is at least no serious distortion of the truth through a more or less “close translation.”
There is also a textual problem in this verse. Some ancient manuscripts read “and no one has ever gone up to heaven except the one who came down from heaven, the Son of Man, who is in heaven.” JB, NEB, Mf t , and Zü r all accept the words “who is in heaven” as a part of the original text. NA B , however, places them in brackets, and Zü r and NEB both have a footnote, indicating the absence of these words from some manuscripts. The other translations which include them give no note (except Segond). This phrase occurs in a few Greek manuscripts, in the Latin, and in a few Syriac versions. The textual evidence in its favour is weak and diverse. Yet the phrase is so difficult that it is hard to see why any manuscript would include it if it were not an original part of the text. The UB S Committee on the Greek text supports the shorter reading because of the strong manuscript evidence. They believe that the words “who is in heaven” were added later as a further Christological interpretation of this verse. [1]
TEV Today’s English Version
J B Jerusalem Bible
NEB New English Bible
Mft Moffatt
Zür Zürcher Bibel
NAB New American Bible
Zür Zürcher Bibel
NEB New English Bible
UBS United Bible Societies
[1] Newman, B. M., & Nida, E. A. (1993). A handbook on the Gospel of John. UBS Handbook Series (84–85). New York: United Bible Societies.