And, in speaking of the new birth to Nicodemus, Jesus said, "We're talking about things which we have seen and we are testifying about things that we know about." So Jesus and His disciples had seen people born again and knew of people who had been born again at that time. Jesus then went on and explained how folks were born again in the words recorded in verses 14-21.
He's talking about the same folks that John speaks of in verses 12 and 13 of chapter 1 of his Gospel. There he speaks of those who had been born of God because they had received Jesus and believed in His name.
This also agrees with John's explanation of the new birth in
1 John 5:1 where he writes, "If you believe that Jesus is the Christ --that he is God's Son and your Savior-- then you are a child of God."