He that cometh from above is above all: he that is of the earth is earthly, and speaketh of the earth: he that cometh from heaven is above all.The Greek word (ἄνωθεν) that is translated as "from above" here is the same one that is rendered "again" in v. 3: "Except a man be born again [or 'born from above'], he cannot see the kingdom of God." (A similar sort of polysemy exists in English, as when we speak of doing something "all over again," "from the top.") The two different translations seem right in their respective contexts, since Nicodemus's reply ("Can he enter a second time into his mother's womb?") clearly has reference to being born again, whereas v. 31 is about "he that cometh from heaven." Nevertheless, v. 31 is almost certainly intended to allude both to "he that came down from heaven" (v. 13) and to the importance of being "born again" (v. 3).
A strictly literal translation would be "he that is from the earth, is from the earth, and speaketh from the earth" ("ἐκ τῆς γῆς," repeated three times), and the immediate context suggests that Jesus is being contrasted with John. As great as John is, he is still "from the earth" and speaks from that perspective.
Matthew quotes Jesus as saying, "Among them that are born of women there hath not risen a greater than John the Baptist: notwithstanding he that is least in the kingdom of heaven is greater than he" (Matthew 11:11). While I use Matthew with caution, considering it the least reliable of the Gospels, I consider this to be an authentic saying, on the grounds that Matthew would never have put such problematic (from the Matthean perspective) words in Jesus' mouth. While Matthew and Luke deny that Jesus had a biological father, their fanciful nativity stories would still include Jesus "among them that are born of women" -- and thus, according to Matthew 11:11, not greater than John the Baptist.
The Fourth Gospel shows us the correct interpretation of Matthew 11:11. Jesus was "from heaven" not because there was anything unusual about the circumstances of his biological birth -- he was a son of man, born of a woman -- but because he had been born again, born from above, born of the spirit, born of God. Though John was the one who recognized this transformation in Jesus -- seeing the Spirit of God descend on him and stay -- he was apparently not "born again" himself. John represents the highest peaks of holiness attainable without being born again. In a way, we might think of him as Jesus' "Virgil" -- his role being analogous to the one given to that poet in Dante's Comedy. (In fact, if we substitute Virgil's name for John's in Matthew 11:11, doesn't it ring just as true? It is perhaps what Jesus would have said had he been born a Roman rather than a Jew.)
 And what he hath seen and heard, that he testifieth; and no man receiveth his testimony.  He that hath received his testimony hath set to his seal that God is true.This echoes what Jesus says to Nicodemus: "Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness" (v. 11). "Testimony" and "witness" translate the same Greek word (μαρτυρίαν). This is further evidence that this passage is the author writing in his own voice; John the Baptist was not privy to Jesus' conversation with Nicodemus and could not have quoted it.
Immediately after "no man receiveth his testimony" comes a statement about "he that hath received his testimony"! How to resolve this obvious contradiction?
The most natural interpretation is to read v. 32 as a bit of commonplace hyperbole, actually meaning something like "very few receive his testimony." Greek reference works (qv) seem not to countenance such a reading, though. We are told that οὐδείς (translated as "no man," but more literally "no one and nothing") "'shuts the door' objectively and leaves no exceptions," that it "categorically excludes, declaring as a fact that no valid example exists." A negation that was qualified in any way would use μηδείς rather than οὐδείς. In other words, v. 32 means "no one at all receives his testimony" -- not the sort of expression that would be used hyperbolically.
Could v. 33 be counterfactual ("Anyone who received his testimony would be setting to his seal...")? I don't think so, since it is in the indicative mood. What then can it mean?
"Set to his seal that God is true" means "confirmed that God is truthful or trustworthy." So perhaps the meaning is that Jesus' claims are so extraordinary that no one at all takes Jesus' word for it that they are true. Whoever seems to be "receiving his testimony" -- i.e., believing what he says because he says it -- actually believes not because of Jesus' testimony but because God has confirmed the truth to them directly. "He that believeth on me, believeth not on me, but on him that sent me." (John 12:44; cf. Matthew 10:40, Mark 9:37, and Luke 9:48, all of which have "receive" in place of "believe on").
 For he whom God hath sent speaketh the words of God: for God giveth not the Spirit by measure unto him.  The Father loveth the Son, and hath given all things into his hand."My doctrine is not mine, but his that sent me" (John 7:16). The idea of Jesus having been "sent" by God is characteristic of the Fourth Gospel, occurring only in a few isolated passages of the Synoptics.
For "not . . . by measure" read "without measure." Others have been inspired, but only Jesus was fully inspired, to the extent that his words were the words of God.
 He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him.The first clause is simply a restatement of v. 16. The final part, about "the wrath of God" seems out of place. It is the only occurrence of the word "wrath" (ὀργὴ) in the Fourth Gospel, and one of only five occurrences in the Gospels as a whole. Mark 3:5 has Jesus look around with anger at those who would criticize him for healing a man on the Sabbath, and Luke 21:23 has Jesus speak of "great distress in the land, and wrath upon this people" as part of an apocalyptic prophecy. The other two instances quote John the Baptist execrating the Pharisees and Sadducees who had come to be baptized: "O generation of vipers, who hath warned you to flee from the wrath to come?" (Matthew and Luke 3:7).
All in all, this does seem to be language more typical of John the Baptist than of the author of the Fourth Gospel, and some commentators do read all of vv. 27-36 as a quotation from John. For the reasons given above, I stand by my opinion that vv. 31-36 are the author's own words and not John's, but the context does render probable an allusion to John and his distinctive way of speaking. Perhaps we should imagine quotation marks around the last clause. He that believeth not the Son shall not see life; but -- as John, speaking "from the earth," would put it -- "the wrath of God abideth on him." This is the sort of thing I often do in my own writing -- quoting or alluding to someone made salient by context even when it means using phraseology with which I would normally have reservations -- so I can easily imagine the evangelist doing the same thing.