Time, space, matter. Each of those consist of three, time consists of past, present, and future. Space consists of height, depth, and breadth. Matter consists of solids, gas, and liquid. Then there's mankind.
Mankind consists of body, soul, and spirit.
Man includes man, woman, and child. The Christian life consists of faith, hope, and love. Knowing, brethren beloved, your election of God. Paul is telling these Believers that they are among the elect who were chosen in eternity past by God. The Holy Spirit gave Paul the insight that enabled him to know that these people were chosen by God.