The three persons of the Trinity do not exist for human benefit but rather because the Godhead is an inherent eternal attribute of the one eternal living God.
Consider these verses of Holy Scripture (all taken from the Douay-Rheims):
-“In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by him: and without him was made nothing that was made.” (John 1:1-3)
-“For in him [Jesus Christ] dwelleth all the fulness of the Godhead corporeally” (Col 2:9)
-“Thus saith the Lord thy redeemer, and thy maker, from the womb: I am the Lord, that make all things, that alone stretch out the heavens, that establish the earth, and there is none with me.” (Isa. 44:24)
Now isn’t that interesting: John states that the Word was with God, and even was God, and that the Word was a part of the creation of all created things! Yet Isaiah records God telling him that He created the heavens and earth all alone. The answer to how both of these can be true is to recognize that God is not like us, but rather can exist in three distinct Persons all at the same time, while still being one Being.
Now it’s important to recognize that the Word of God has not been eternally human, but rather: “the Word was made flesh, and dwelt among us, (and we saw his glory, the glory as it were of the only begotten of the Father,) full of grace and truth.” (John 1:14) And “Therefore because the children are partakers of flesh and blood, he also himself in like manner hath been partaker of the same: that, through death, he might destroy him who had the empire of death, that is to say, the devil…”(Heb 2:14). So before the fall all three Persons of the Trinity were spirit (cf. John 4:24), but after the fall one of the Three took up a human body for the redemption of human beings, and this body was glorified at His resurrection- and that fleshly temple of the Word stands to this day (cf. John 2:19)
