Nicene and Post-Nicene Fathers, Vol. III:
Doctrinal Treatises of St. Augustin: Chapter 7

Chapter 7.—God is a Trinity, But Not Triple (Triplex).

But God is truly called in manifold ways, great, good, wise, blessed, true, and whatsoever other thing seems to be said of Him not unworthily: but His greatness is the same as His wisdom; for He is not great by bulk, but by power; and His goodness is the same as His wisdom and greatness, and His truth the same as all those things; and in Him it is not one thing to be blessed, and another to be great, or wise, or true, or good, or in a word to be Himself.

9. Neither, since He is a Trinity, is He therefore to be thought triple (triplex) 615 otherwise the Father alone, or the Son alone, will be less than the Father and Son together. Although, indeed, it is hard to see how we can say, either the Father alone, or the Son alone; since both the Father is with the Son, and the Son with the Father, always and inseparably: not that both are the Father, or both are the Son; but because they are always one in relation to the other, and neither the one nor the other alone. But because we call even the Trinity itself God alone, although He is always with holy spirits and souls, but say that He only is God, because they are not also God with Him; so we call the Father the Father alone, not because He is separate from the Son, but because they are not both together the Father.

Footnotes

[The Divine Unity is trinal, not triple. The triple is composed of three different substances. It has parts, and is complex. The trinal is without parts, and is incomplex. It denotes one simple substance in three modes or forms. “We may speak of the trinal, but not of the triple deity.” Hollaz, in Hase’s Hutterus, 172.—W.G.T.S.]