Objection 1. It would seem that essential names, as the name "God," should not be predicated in the singular of the three persons, but in the plural. For as "man" signifies "one that has humanity," so God signifies "one that has Godhead." But the three persons are three who have Godhead. Therefore the three persons are "three Gods."
Objection 2. Further, Genesis 1:1, where it is said, "In the beginning God created heaven and earth," the Hebrew original has "Elohim," which may be rendered "Gods" or "Judges": and this word is used on account of the plurality of persons. Therefore the three persons are "several Gods," and not "one" God.
Objection 3. Further, this word "thing" when it is said absolutely, seems to belong to substance. But it is predicated of the three persons in the plural. For Augustine says (De Doctr. Christ. i, 5): "The things that are the objects of our future glory are the Father, Son and Holy Ghost." Therefore other essential names can be predicated in the plural of the three persons.
Objection 4. Further, as this word "God" signifies "a being who has Deity," so also this word "person" signifies a being subsisting in an intellectual nature. But we say there are three persons. So for the same reason we can say there are "three Gods."
On the contrary, It is said (Deuteronomy 6:4): "Hear, O Israel, the Lord thy God is one God."
I answer that, Some essential names signify the essence after the manner of substantives; while others signify it after the manner of adjectives. Those which signify it as substantives are predicated of the three persons in the singular only, and not in the plural. Those which signify the essence as adjectives are predicated of the three persons in the plural. The reason of this is that substantives signify something by way of substance, while adjectives signify something by way of accident, which adheres to a subject. Now just as substance has existence of itself, so also it has of itself unity or multitude; wherefore the singularity or plurality of a substantive name depends upon the form signified by the name. But as accidents have their existence in a subject, so they have unity or plurality from their subject; and therefore the singularity and plurality of adjectives depends upon their "supposita." In creatures, one form does not exist in several "supposita" except by unity of order, as the form of an ordered multitude. So if the names signifying such a form are substantives, they are predicated of many in the singular, but otherwise if they adjectives. For we say that many men are a college, or an army, or a people; but we say that many men are collegians. Now in God the divine essence is signified by way of a form, as above explained (2), which, indeed, is simple and supremely one, as shown above (3, 7; 11, 4). So, names which signify the divine essence in a substantive manner are predicated of the three persons in the singular, and not in the plural. This, then, is the reason why we say that Socrates, Plato and Cicero are "three men"; whereas we do not say the Father, Son and Holy Ghost are "three Gods," but "one God"; forasmuch as in the three "supposita" of human nature there are three humanities, whereas in the three divine Persons there is but one divine essence. On the other hand, the names which signify essence in an adjectival manner are predicated of the three persons plurally, by reason of the plurality of "supposita." For we say there are three "existent" or three "wise" beings, or three "eternal," "uncreated," and "immense" beings, if these terms are understood in an adjectival sense. But if taken in a substantive sense, we say "one uncreated, immense, eternal being," as Athanasius declares.
Reply to Objection 1. Though the name "God" signifies a being having Godhead, nevertheless the mode of signification is different. For the name "God" is used substantively; whereas "having Godhead" is used adjectively. Consequently, although there are "three having Godhead," it does not follow that there are three Gods.
Reply to Objection 2. Various languages have diverse modes of expression. So as by reason of the plurality of "supposita" the Greeks said "three hypostases," so also in Hebrew "Elohim" is in the plural. We, however, do not apply the plural either to "God" or to "substance," lest plurality be referred to the substance.
Reply to Objection 3. This word "thing" is one of the transcendentals. Whence, so far as it is referred to relation, it is predicated of God in the plural; whereas, so far as it is referred to the substance, it is predicated in the singular. So Augustine says, in the passage quoted, that "the same Trinity is a thing supreme."
Reply to Objection 4. The form signified by the word "person" is not essence or nature, but personality. So, as there are three personalities--that is, three personal properties in the Father, Son and Holy Ghost--it is predicated of the three, not in the singular, but in the plural.
Source: Summa Theologica (New Advent)