Though the Spirit be called Lord, three Lords are not thereby implied; inasmuch as two Lords are not implied by the fact that the Son in the same manner as the Father is called Lord in many passages of Scripture; for Lordship exists in the Godhead, and the Godhead in Lordship, and these coincide without division in the Three Persons.
104. But perhaps, again, you may say: If I call the Spirit Lord, I shall set forth three Lords. Do you then when you call the Son Lord either deny the Son or confess two Lords? God forbid, for the Son Himself said: “Do not serve two lords.” But certainly He denied not either Himself or the Father to be Lord; for He called the Father Lord, as you read: “I thank You, O Father, Lord of heaven and earth.” And the Lord spoke of Himself, as we read in the Gospel: “You call Me Master and Lord, and you do well, for so I am.” But He spoke not of two Lords; indeed He shows that He did not speak of two Lords, when He warns them: “Do not serve two lords.” For there are not two Lords where the Lordship is but one, for the Father is in the Son and the Son in the Father, and so there is one Lord.
105. Such, too, was the teaching of the Law: “Hear, O Israel, the Lord your God is one Lord,” that is, unchangeable, always abiding in unity of power, always the same, and not altered by any accession or diminution. Therefore Moses called Him One, and yet also relates that the Lord rained down fire from the Lord. The Apostle, too, says: “The Lord grant unto him to find mercy of the Lord.” The Lord rains down from the Lord; the Lord grants mercy from the Lord. The Lord is neither divided when He rains from the Lord, nor is there a separation when He grants mercy from the Lord, but in each case the oneness of the Lordship is expressed.
106. In the Psalms, too, you find: “The Lord said unto my Lord.” And he did not therefore deny that the Father was his Lord, because he spoke of the Son as his Lord; but therefore called the Son his Lord, that you might not think Him to be the Son, but the Lord of the prophet, as the Lord Himself showed in the Gospel, when He said: “If David in the Spirit called Him Lord, how is he his Son?” David, not the Spirit, calls Him Lord in the Spirit. Or if they falsely infer from this that the Spirit called Him Lord, they must necessarily by a like sacrilege seem to assert that the Son of God is also the Son of the Spirit.
107. So, as we do not say that there are two Lords, when we so style both the Father and the Son, so, too, we do not say that there are three Lords, when we confess the Spirit to be Lord. For as it is profane to say that there are three Lords or three Gods, so, too, is it utter profanity to speak of two Lords or two Gods; for there is one God, one Lord, one Holy Spirit; and He Who is God is Lord, and He Who is Lord is God, for the Godhead is in the Lordship, and the Lordship is in the Godhead.
108. Lastly, you have read that the Father is both Lord and God: “O Lord my God, I will call upon You, hear me.” You find the Son to be both Lord and God, as you have read in the Gospel, that, when Thomas had touched the side of Christ, he said, “My Lord and my God.” So in like manner as the Father is God and the Son Lord, so too the Son is God and the Father Lord. The holy designation changes from one to the other, the divine nature changes not, but the dignity remains unchangeable. For they are not [as it were] contributions gathered from bounty, but free-will gifts of natural love; for both Unity has its special property, and the special properties are bound together in unity.
Source: On the Holy Spirit (New Advent)