We must conclude from all we have said that in the Godhead there is something threefold which is not opposed to the unity and simplicity of the divine essence. We must acknowledge that God is, as existing in His nature, and that He is known and loved by Himself.
But this occurs otherwise in God than in us. Man, to be sure, is a substance in his nature; but his actions of knowing and loving are not his substance. Considered in his nature, man is indeed a subsisting thing; as he exists in his mind, however, he is not a subsisting thing, but a certain representation of a subsisting thing; and similarly with regard to his existence in himself as beloved in lover. Thus man may be regarded under three aspects: that is, man existing in his nature, man existing in his intellect, and man existing in his love. Yet these three are not one, for man’s knowing is not his existing, and the same is true of his loving. Only one of these three is a subsisting thing, namely, man existing in his nature.
In God, on the contrary, to be, to know, and to love are identical. Therefore God existing in His natural being and God existing in the divine intellect and God existing in the divine love are one thing. Yet each of them is subsistent. And, as things subsisting in intellectual nature are usually called persons in Latin, or hypostases in Greek, the Latins say that there are three persons in God, and the Greeks say that there are three hypostases, namely, the Father, the Son, and the Holy Spirit.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Trinity of Divine Persons and the Unity of the Divine Essence, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
All scripture is given by inspiration of God and is profitable for doctrine, reproof, correction, and instruction in righteousness (2 Timothy 3:16).