Here's the NET footnote of John 3:13. Quite comprehensive and gives the different readings (for those who have an interest in textual criticism):
tc Most witnesses, including a
few important ones (A[*] Θ Ψ
050 ƒ1,
13 Û latt syc,p,h), have at the end of this verse "the one
who is in heaven" (ὁ ὢν ἐν τῷ
οὐρανῷ, ho on en to ourano). A few others have
variations on this phrase, such as "who was in heaven" (e syc), or "the one who is from heaven" (0141 pc
sys). The witnesses normally
considered the best, along with several others, lack the phrase in its entirety
(î66,
75 א B
L T Ws 083 086 33 1241 pc co). On
the one hand, if the reading ὁ ὢν ἐν τῷ οὐρανῷ
is authentic it may suggest that while Jesus was speaking
to Nicodemus he spoke of himself as in heaven even while he was on earth. If
that is the case, one could see why variations from this hard saying arose: "who
was in heaven," "the one who is from heaven," and omission of the
clause. At the same time, such a saying could be interpreted (though with
difficulty) as part of the narrator's comments rather than Jesus' statement to
Nicodemus, alleviating the problem. And if v. Joh 3:13 was viewed in early times as
the evangelist's statement, "the one who is in heaven" could have crept into the
text through a marginal note. Other internal evidence suggests that this saying
may be authentic. The adjectival participle, ὁ
ὤν, is used in the Fourth Gospel more than any other NT
book (though the Apocalypse comes in a close second), and frequently with
reference to Jesus (Joh
1:18; Joh
6:46; Joh
8:47). It may be looking back to the LXX of Exo 3:14 (ἐγώ
εἰμι ὁ ὤν). Especially since this exact construction is not
necessary to communicate the location of the Son of Man, its presence in many
witnesses here may suggest authenticity. Further, John uses the singular of
οὐρανός (ouranos, "heaven")
in all 18 instances of the word in this Gospel, and all but twice with the
article (only Joh 1:32
and Joh 6:58
are anarthrous, and even in the latter there is significant testimony to the
article). At the same time, the witnesses that lack this clause are very weighty
and must not be discounted. Generally speaking, if other factors are equal, the
reading of such MSS should be preferred. And internally, it could be argued that
ὁ ὤν is the most concise way to
speak of the Son of Man in heaven at that time (without the participle
the point would be more ambiguous). Further, the articular singular οὐρανός is already used twice in this
verse, thus sufficiently prompting scribes to add the same in the longer
reading. This combination of factors suggests that ὁ ὢν
ἐν τῷ οὐρανῷ is not a genuine Johannism. Further intrinsic
evidence against the longer reading relates to the evangelist's purposes: If he
intended v. Joh 3:13
to be his own comments rather than Jesus' statement, his switch back to Jesus'
words in v. Joh 3:14
(for the lifting up of the Son of Man is still seen as in the future) seems
inexplicable. The reading "who is in heaven" thus seems to be too hard. All
things considered, as intriguing as the longer reading is, it seems almost
surely to have been a marginal gloss added inadvertently to the text in the
process of transmission. For an argument in favor of the longer reading, see
David Alan Black, "The Text of Joh 3:13, " GTJ 6 (1985): 49-66.
