In all three texts, the consistent truth Jesus was referring to is that the Kingdom of God cannot be seen with our literal eyes. He worded his statements so as to get them thinking about that.
He did not come out with a blunt, "Nobody can see the Kingdom of God with their physical eyes". He spoke to people in such a way as to challenge their established concepts (where they were wrong). The error with all those people was that they supposed the Kingdom of God would be like the kingdom of Israel, or the kingdom of Greece, or the empire of Rome - with an appointed, visible king, emperor, or ruler. They couldn't get beyond mere human ideas about a kingdom. They thought it would have territory - a dominion with borders - and armies to defend or expand those borders. The Israelites thought the Messiah would come with great power and pomp, to overthrow the Roman empire and re-establish the throne of David, with its base in Jerusalem.
In the first text, Jesus answered those who despised him (for appearing to lack authority to claim to be the Messiah). They were looking in the wrong place because this Kingdom of God is based in the heart of humans who are subjects of the King (himself). Thus, they would never see it till they repented and understood him to be the promised King.
In the second text, Nicodemus believed God was "with" Jesus, who wanted to move Nicodemus on in his understanding. So, he challenged him to consider that he would never see the Kingdom of God unless he was "born again" (or, "born from above"). Nicodemus remained in a literal mode of thinking, and just did not get it. Jesus was not speaking of literal birth any more than he was speaking of an earthly Kingdom. He spoke of spiritual new birth and a spiritual Kingdom. Jesus went on to explain that "as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up, that whosoever believeth in him should not perish, but have eternal life." That eternal life is the portion of all who are brought into the Kingdom of God by the invisible working of the Holy Spirit. We have all been 'bitten' by the 'snake' of sin, and only by looking in faith to Christ raised up can our sin be dealt with, so that we might enter into the Kingdom of God.
The third text points to an event that followed Jesus' statement here. Some of the disciples were taken up into a mountain where they saw Jesus glorified, supernaturally, Moses and Elijah appearing beside him. For that brief time, they got a glimpse of the glory of the King of God's Kingdom. Now, those who don't think that is the meaning, might point to an event later on, when they saw the resurrected Jesus. Either way, a privileged few got to see beyond the humanity of Christ, to his celestial glory.
As Jesus is the King of God's Kingdom (which is not earthly - it's heavenly - even though it will have permanent effect on the New Earth created after the Day of Judgment), the only way to 'enter' into that Kingdom is to become a subject of its King. Once the glorified Christ reigns in our hearts, minds, and lives, we "see" what this Kingdom means. When we step out of time and enter eternity, we will be in the heavenly realm where all heaven and its inhabitants are "in" that Kingdom.