John 3 is mostly an interlude, having parallels with 1 John. The table shows parallels between John 3 and 1 John.
I don't believe John 3:11-21 are Jesus' words or that John 3:31-36 are the words of John the Baptist. Rather, it is the narration of the evangelist/author of John and 1 John. Important clues, in addition to the parallels with 1 John, are the plurals of John 3:11-12.
The indication that an interlude narration begins in verse 11 is
(1) “you” is in the plural both in verses 11 and 12 (although Jesus was only speaking to Nicodemus)
(2) “we speak what we know and testify what we have seen” contains 4 plural verbs
(3) “our” is plural in reference to “our testimony”
According to UBS Handbooks for New Testament (20 Vols.) on John 3:11, “The shift from singular to plural should be carefully noted. The verse begins with the first person singular (I) addressing the second person singular (you). The shift is then made to the first-person plural (we... our) addressing the second-person plural (none of you). A number of theories exist as to why this shift is made, but the most probable solution is that John has shifted the time perspective from Jesus' day to the time in which he writes his Gospel. If so, then “we” represents the Christian believers of John's own day who are in dialogue with the Jews represented by “you (plural).”
In John 3:12, “you” throughout the verse is plural, so it must not only be addressed to Nicodemus. (UBS Handbooks for New Testament (20 Vols.))
With respect to John 3:31-36, the UBS Handbooks for New Testament (20 Vols.) notes, "It is possible that these words are the comments of the author of the Gospel. This is the opinion held by TEV, NEB, RSV, and Gdsp. If this is the case, there is a parallel between verses 14-21 (or 16-21) and the present passage. That is, the earlier section represents the author's commentary on Jesus' dialogue with Nicodemus, while this passage serves as a commentary on the relationship between Jesus and John the Baptist.”
Red-letter Bibles need a reprint.