Where are both Eliya and Enoch presently because the Bible say that only Son of God has ascended to heaven (John 3:13)?
You are very observant in asking about John 3:13. I will start by confusing you even more. The verse says, “No man has ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven.” If this is Jesus speaking of himself, then when did he ascend up to heaven before this time? And how could he (the Son of man) be speaking on earth and claim to be in heaven at the same time?
I believe that verse 13 was added by John as a comment on what Jesus had said previously, just as I believe that John 3:16 (perhaps the most famous passage in the New Testament) is not the words of Jesus but commentary by John. If that is so, then my two questions are easily answered. Jesus asked about heavenly things (verse 12), and John comments that he would know about heavenly things because he is the one who has come down from heaven, ascended to heaven, and is (at the time John is writing this and ever since) in heaven. In that way Jesus need not be on earth and in heaven at the same time, and had already ascended after his death.
But that doesn’t totally answer your question about Eliya (Elijah) and Enoch. The most common answer would be that people do not go directly to heaven/hell after they die, or are taken from the earth without dying. Instead they are in some sort of waiting area (called “Abraham’s bosom” or a place of torment in Luke 16). This would fit with the idea that the final judgement will not be until Jesus comes again (Matthew 25:31 and following). When the Bible talks about going up into heaven it could be speaking of three different things. One would be simply flying up into the area where the birds fly. Eliya apparently went higher than that. The second heaven would be the realm of the stars. Eliya went at least that far, probably. Whether God took him to the third heaven, the abode of God, is simply not stated. As for Enoch, we don’t know how he was taken, whether there was any sort of “ascension” or not. So Enoch may not be involved in this question at all. Jesus (or John) may be implying that Enoch and Eliya did not die, but have yet to reach the ultimate abode of God.
I don’t know exactly what is meant by the passage. Was John, although inspired by the Holy Spirit, expressing his own opinion? Are Enoch and Eliya in some holding area, awaiting final judgement? If this verse is the words of Jesus, is he talking about Eliya rather than himself? I am not sure. I do know that it is not something I absolutely have to understand before I can be saved.
Sorry if I have not been of much help.