Regarding the words stated at John 3:13 I don't know if Jesus ever said them. What I do know is that according the biblical book named According to John, Jesus said those words. But, the book might be wrong in attributing those words to Jesus. Even if Jesus actually said those words, note that according to the narrative of which John 3:13 is a part, Jesus had not yet died and not yet been resurrected. Notice that verse says Jesus (the pre-resurrection Jesus according to the narrative) says the Son of Man is the one who has ascended into heaven. How can one explain that if one believes that Jesus is the son of man? Does that mean that according to narrative Jesus claimed that Jesus ascended to heaven before Jesus died on Earth (perhaps even making trips to Earth prior to being conceived in the womb of Mary)? Some Christians think so, but it is not clear to me that the Bible teaches such. Even if it does teach such, the OT claims that angels from heaven came down to Earth and later returned to heaven. Are these biblical claims examples of the Bible contradicting itself?
A number of scholars say that the historical Jesus did not consider himself to be the son of man, but that later Christians (prior to the NT gospel account being written) came to believe Jesus was the son of man. Many current Christians believe that Jesus is the son of man and that he claimed to be the son of man, but they might be wrong in believing such.