7 But what means this: “They contained two or three metretæ apiece”? This phrase certainly conveys to us a mysterious meaning. For by “metretæ” he means certain measures, as if he should say jars, flasks, or something of that sort. Metreta is the name of a measure, and takes its name from the word “measure.” For μέτρον is the Greek word for measure, whence the word “metretæ” is derived. “They contained,” then, “two or three metretæ apiece.” What are we to say, brethren?
If He had simply said “three apiece,” our mind would at once have run to the mystery of the Trinity. And, perhaps, we ought not at once to reject this application of the meaning, because He said, “two or three apiece;” for when the Father and Son are named, the Holy Spirit must necessarily be understood. For the Holy Spirit is not that of the Father only, nor of the Son only, but the Spirit of the Father and of the Son. For it is written, If any man love the world, the Spirit of the Father is not in him. And again, “Whoso has not the Spirit of Christ is none of His.” The same, then, is the Spirit of the Father and of the Son.
Therefore, the Father and the Son being named, the Holy Spirit also is understood, because He is the Spirit of the Father and of the Son. And when there is mention of the Father and Son, “two metretæ,” as it were, are mentioned; but since the Holy Spirit is understood in them, “three metretæ.” That is the reason why it is not said, “Some containing two metretæ apiece, others three apiece;” but the same six water-pots contained “two or three metretæ apiece.” It is as if he had said, When I say two apiece, I would have the Spirit of the Father and of the Son to be understood together with them; and when I say three apiece, I declare the same Trinity more plainly.
Source: Tractates on the Gospel of John (New Advent)