To those who object that Catholics, when they ascribe Godhead to the Holy Spirit, introduce three Gods, it is answered, that by the same argument they themselves bring in two Gods, unless they deny Godhead to the Son; after which the orthodox doctrine is set forth.
92. But what do you fear? Is it that which you have been accustomed to carp at? Lest you should make three Gods. God forbid; for where the Godhead is understood as one, one God is spoken of. For neither when we call the Son God do we say there are two Gods. For if, when you confess the Godhead of the Spirit, you think that three Gods are spoken of, then, too, when you speak of the Godhead of the Son because you are not able to deny it, you bring in two Gods. For it is necessary according to your opinion, if you think that God is the name of one person, not of one nature, that you either say that there are two Gods, or deny that the Son is God.
93. But let us free you from the charge of ignorance, though we do not excuse you from fault. For according to our opinion, because there is one God, one Godhead and oneness of power is understood. For as we say that there is one God, confessing the Father, and not denying the Son under the true Name of the Godhead; so, too, we exclude not the Holy Spirit from the Unity of the Godhead, and do not assert but deny that there are three Gods, because it is not unity but a division of power which makes plurality. For how can the Unity of the Godhead admit of plurality, seeing that plurality is of numbers, but the Divine Nature does not admit numbers?
Source: On the Holy Spirit (New Advent)