The Testator, the Third Member of the Godhead Born Also

Testator or Holy Ghost, born.  It was necessary that the third member of the Godhead also take upon himself mortality to fill the measure of his creation.  This office of Testator is without beginning of days nor end of years, without father, mother, but the individual (Joseph Smith) who occupied that office for this earth from the foundation thereof begotten by parents who were mortal (since there is but one Only Begotten who is Jesus Christ). 

The Revelations of Jesus Christ

Section 161

Thus saith the Lord, it was that Joseph, our beloved Joseph was the Testator born in the flesh, that he might also fulfill the measure of the sons of God, that he might fulfill the measure of his creation, because it is that spirit united with flesh and bones bringeth a fullness of joy.
2 And thus it was necessary that he whose splendor, radiance and intelligence was greater or equal to the other of our Father’s sons, even the Christ, with whom he is One forever and ever, saith the Father, yea, it was necessary that he be obedient unto the law and descend below all things, that he too might be in all things and through all things the light of truth, with the Father and His Christ, being clothed with flesh and blood, he did verily complete his mission.
3 And He lived for glory and verily died for glory, and sealed his testimony with his blood, that this word should become a law unto the Church forever, unchanging. He is the Holy Ghost, and this was his office, saith the Lord, before the world was organized.
4 Even so. Amen.

"Everlasting covenant between three personages...God the first, the Creator; God the second, the Redeemer; and God the third, the witness or Testator."  Joseph Smith, TPJS p.190