"That God is a Trinity also implies that there is eternal difference in God. Some of Jesus's statements about his relation to the Father are given in order to establish just this point. Jesus says that he has "received" from the Father and that things have been "delivered" to him "only to show that He is not the Father, but the Father's Word, and the Eternal Son, who because of His likeness to the Father, has eternally what He has from him, and because He is the Son, has from the Father what He has eternally" (Discourses 3.36)." - Leithart, Athanasius, p83.
God could not have not been eternally Triune and then eternally Triune. If there is an eternal Father, He must be eternally Father with a Son, and so forth. If eternally Father and Son, then eternally Triune.