What is these3? ​​​​​​​

These3 is about Jesus Christ, and how we can know Him. 
Then, how we can advance to a more complete knowledge of Him.
Then, how we can bring many sons and daughters of God into the Kingdom rule of God.

The mysteries of the New Testament center around 2 advents of Jesus, the Messiah. God is using both of Jesus' comings, or advents, to usher in the kingdom rule of the King of Heaven.
The first coming was to break the power of Satan, sin, and death. He accomplished this by His finished work of the cross, and His subsequent resurrection from the grave, followed by His ascension to Heaven.
The Second Coming will bring about the crushing of God’s enemies and the fulfillment of the OT expectations of a time of peace under the powerful rule of God over the earth.
In between the two comings (where we all live)  is a time of mercy for God’s grace and peace to be preached to the nations. It is a time where God pours all of his administration for the earth into the church, to display His glory to the earth.
Regardless of how little we personally enter into this reality, this is God's progressive plan. And by grace, we can jump right in and participate immediately. (refer to the Lord's prayer)