No man has ascended to heaven.

Jesus said this just before the famous passage that nearly all have heard (Jn.3:16)

John 3:12-14 Jesus explains “If I have told you earthly things and you do not believe, how will you believe if I tell you heavenly things? No one has ascended to heaven but He who came down from heaven, that is, the Son of Man who is in heaven.”

As with any scripture we need to read it in its full context and not let a certain portion be lifted out that overrides the actual meaning for our interpretation. What Jesus is saying is that No man, is qualified to speak of heavenly things, we know this is part of the answer because of the verse before says this v.12. "If I have told you earthly things, and you do not believe, how shall you believe, if I tell you of heavenly things?”

For anyone to speak of these things in heaven requires one of seeing them; and as no one has ascended into heaven and returned, so no one is qualified to speak of them but one, He who came down from heaven to earth. This does not mean that no one had GONE to heaven, for Enoch and Elijah had been brought there without dying (Gen 5:24; compare Heb. 11:5; 2 Kings 2:11).

What it means is that no one had ascended and "returned," to be qualified to speak of the things there. There is only one, the one called the Son who descended.

John 6:62 What then if you should see the Son of Man ascend where He was before?" In other words not even Jesus as a man yet ascended to where he came from.

[But he that came down ...] The Lord Jesus. He is represented as coming down, because, being equal with God, he took upon himself our nature, John 1:14; Phil 2:6-7. He is represented as "sent" by the Father, John 3:17,34; Gal 4:4; 1 John 4:9-10.(from Barnes' Notes)

Has ascended or "has been there." Jesus says that no one has been in heaven except the Son of man who came down out of heaven; because no man could be in heaven without having ascended thither. (from Vincent's Word Studies of the New Testament)

Jesus’ point is that no earthly teacher can teach about heaven, because none have ascended there and returned to tell you the things I have, because I’m in heaven. No one has ever gone to heaven and returned. But the Son of Man has come down from heaven.” (New Living Translation)

here is the point we need to understand,  one has ascended to heaven - Jesus did not ascend to heaven yet, until after his resurrection. So this at the time would apparently include him as well. but he couples it with but He who came down from heaven.

The word (no) “man” in the light of the context, no doubt refers to the human body, and in this sense, no man has ascended to heaven in our bodily form, the resurrection of the body, the transformation has not yet taken place.

[ascended up] in the Greek: is anabaino ( which means to go up of own power. This  might clear up the question on others taken up such as was Enoch and Elijah. At  His ascension that the righteous who were in paradise (Abrahams bosom) were transferred to the third heaven- where paradise now is. (Eph 4:8-10)

Has ascended or "has been there, or who is in heaven."  The emphasis is on He who came down from heaven, that is, the Son of Man who is in heaven.” In other words it is his place of origin, where he resides, of which he made plain throughout his ministry. The rest of the discourse is about why God sent his Son.

Then in John 3:31-32 “He who comes from above is above all; he who is of the earth is earthly and speaks of the earth. He who comes from heaven is above all. And what He has seen and heard, that He testifies.” Again referring back to what he first said about ascending and descending.

Proverbs 30:4, the writer asks a series of six questions. The first question is: Who has ascended up into heaven, and descended? The fifth question is what is his name. Jesus by his statement to Nicodemus is identifying himself to Nicodemus as the one it speaks of.

So the meaning of the passage is not of Jesus giving a definitive statement that no one has gone to heaven (ie. Enoch or Elijah).