You are doing violence to the Scriptures when you take statements out of context. The first is Luke 17:21. Jesus is there speaking to the Pharisees and a group of His followers. The Pharisees ask Him when will the Kingdom of God come? The expression, "Kingdom of Heaven" is used exclusively in the gospel of Matthew, by the way. The other three gospels refer to the Kingdom of God. He replies that the Kingdom of God is already among them, (in their midst) meaning Himself as the King--but they do not recognize Him.
The next is John 3:13. Jesus is speaking to Nicodemus, who had come to Him, under cover of darkness, because he feared the Jewish authorities. Nicodemus was a Pharisee who almost certainly would have believed that the righteous dead went to be with Abraham in a beautiful Edenic paradise--one of two sections of sheol (Hebrew for "the place of the dead"--our New Testament translates the original Greek hades as "hell" but it instead, would have been equivalent to sheol--the "place of the dead"). The other section of hades/sheol was the "place of punishment"--the place for the unrighteous dead.
There are two main interpretations of the John 3:13 statement. One has a more Jewish theological cast to it. Jews of Jesus' day, believed that the souls of the righteous dead would have needed to wait for "Messiah ben Joseph" to take them into the presence of God Almighty in heaven (the place where He dwells) when He (the Messiah) was raised from the dead after three days. The writings of a few Talmudic scholars in the centuries preceding Jesus' birth indicate that there would be two Messiahs (instead of one who would come to them twice): Messiah ben Joseph--the "suffering servant" of Isaiah 53 and Messiah ben David--the Conquering King. Two Israeli archeologists recently (at the turn of the 21st century) discovered a stone (they called it the stone of "Gabriel's Revelation") which seems to validate that this was a common belief of Jews prior to Jesus' advent. This is, in my opinion, the main thrust of Jesus' tale of the Rich Man and "the other Lazarus" of Luke 16.
The second, more conventionally Christian, interpretation is that Jesus was saying that none of Nicodemus' teachers have been up to heaven and come back to teach him about it, but that He, had been there. It is preceded by Jesus' rebuke of Nicodemus for not knowing about the "born again" teaching and He says, "...if you don't believe me when I tell you about earthly things, how can you possibly believe if I tell you about heavenly things?" Either interpretation works in the context.
Your third cited statement is, I assume, your rendering of 1 Timothy 6:16. God Almighty (also known as God the Father) is an [immortal] Spirit (cited by Jesus in John 4:24) but humans, since the Fall, are spiritually dead. That is why they must be "born again" (spiritually) "from on high". Some atheist critics have pointed out that Satan was correct--that Adam and Eve did not die the day that they disobeyed God. But, the fact that, when they sinned, they were no longer innocent and realized that they were shamefully naked, means that something spiritual had transpired--their spiritual death--as their bodies and souls were still intact.