The persons of the Godhead do not make multiple gods

Therefore, if they have determined that the Father and the Son must be regarded as one and the same, for the express purpose of vindicating the unity of God, that unity of His is preserved intact; for He is one, and yet He has a Son, who is equally with Himself comprehended in the same Scriptures. Since they are unwilling to allow that the Son is a distinct Person, second from the Father, lest, being thus second, He should cause two Gods to be spoken of, we have shown above that Two are actually described in Scripture as God and Lord. And to prevent their being offended at this fact, we give a reason why they are not said to be two Gods and two Lords, but that they are two as Father and Son; and this not by severance of their substance, but from the dispensation wherein we declare the Son to be undivided and inseparable from the Father,—distinct in degree, not in state. And although, when named apart, He is called God, He does not thereby constitute two Gods, but one; and that from the very circumstance that He is entitled to be called God, from His union with the Father.

Tertullian
Against Praxeas
Chapter XIX

Paul beheld Christ and knew He was God

Him, too, did Paul behold; but yet he saw not the Father. “Have I not,” he says, “seen Jesus Christ our Lord?” Moreover, he expressly called Christ God, saying: “Of whom are the fathers, and of whom as concerning the flesh Christ came, who is over all, God blessed for ever.”

Tertullian
Against Praxeas
Chapter XV

The Word is God of God

Now the Word of life became flesh, and was heard, and was seen, and was handled, because He was flesh who, before He came in the flesh, was the “Word in the beginning with God” the Father, and not the Father with the Word. For although the Word was God, yet was He with God, because He is God of God; and being joined to the Father, is with the Father.

Tertullian
Against Praxeas
Chapter XV

The Son is also God and appeared to Moses

We declare, however, that the Son also, considered in Himself (as the Son), is invisible, in that He is God, and the Word and Spirit of God;

…When as we read in the Gospel, “Moses appeard talking with Jesus”—it is evident that in early times it was always in a glass, (as it were,) and an enigma, in vision and dream, that God, I mean the Son of God, appeared—to the prophets and the patriarchs, as also to Moses indeed himself. And even if the Lord did possibly speak with him face to face, yet it was not as man that he could behold His face, unless indeed it was in a glass, (as it were,) and by enigma.

Tertullian
Against Praxeas
Chapter XIV

The Trinity is three persons but not polytheism

Very well! you say, I shall challenge you to preach from this day forth (and that, too, on the authority of these same Scriptures) two Gods and two Lords, consistently with your views. God forbid, (is my reply). For we, who by the grace of God possess an insight into both the times and the occasions of the Sacred Writings, especially we who are followers of the Paraclete, not of human teachers, do indeed definitively declare that Two Beings are God, the Father and the Son, and, with the addition of the Holy Spirit, even Three, according to the principle of the divine economy, which introduces number, in order that the Father may not, as you perversely infer, be Himself believed to have been born and to have suffered, which it is not lawful to believe, forasmuch as it has not been so handed down. That there are, however, two Gods or two Lords, is a statement which at no time proceeds out of our mouth: not as if it were untrue that the Father is God, and the Son is God, and the Holy Ghost is God, and each is God; but because in earlier times Two were actually spoken of as God, and two as Lord, that when Christ should come He might be both acknowledged as God and designated as Lord, being the Son of Him who is both God and Lord. Now, if there were found in the Scriptures but one Personality of Him who is God and Lord, Christ would justly enough be inadmissible to the title of God and Lord: for (in the Scriptures) there was declared to be none other than One God and One Lord, and it must have followed that the Father should Himself seem to have come down (to earth), inasmuch as only One God and One Lord was ever read of (in the Scriptures), and His entire Economy would be involved in obscurity, which has been planned and arranged with so clear a foresight in His providential dispensation as matter for our faith.  As soon, however, as Christ came, and was recognised by us as the very Being who had from the beginning caused plurality (in the Divine Economy), being the second from the Father, and with the Spirit the third, and Himself declaring and manifesting the Father more fully (than He had ever been before), the title of Him who is God and Lord was at once restored to the Unity (of the Divine Nature), even because the Gentiles would have to pass from the multitude of their idols to the One Only God, in order that a difference might be distinctly settled between the worshippers of One God and the votaries of polytheism.

…I will therefore not speak of gods at all, nor of lords, but I shall follow the apostle; so that if the Father and the Son, are alike to be invoked, I shall call the Father “God,” and invoke Jesus Christ as “Lord.” But when Christ alone (is mentioned), I shall be able to call Him “God,” as the same apostle says: “Of whom is Christ, who is over all, God blessed for ever.” For I should give the name of “sun” even to a sunbeam, considered in itself; but if I were mentioning the sun from which the ray emanates, I certainly should at once withdraw the name of sun from the mere beam. For although I make not two suns, still I shall reckon both the sun and its ray to be as much two things and two forms of one undivided substance, as God and His Word, as the Father and the Son.

Tertullian
Against Praxeas
Chapter XIII

Christ is distinct in personhood from the Father, but not substance

… it was because He had already His Son close at His side, as a second Person, His own Word, and a third Person also, the Spirit in the Word, that He purposely adopted the plural phrase, “Let us make;” and, “in our image;” and, “become as one of us.” For with whom did He make man? and to whom did He make him like? (The answer must be), the Son on the one hand, who was one day to put on human nature; and the Spirit on the other, who was to sanctify man. With these did He then speak, in the Unity of the Trinity, as with His ministers and witnesses.

But all the rest of the created things did He in like manner make, who made the former ones—I mean the Word of God, “through whom all things were made, and without whom nothing was made.” Now if He too is God, according to John, (who says,) “The Word was God,” then you have two Beings—One that commands that the thing be made, and the Other that executes the order and creates. In what sense, however, you ought to understand Him to be another, I have already explained, on the ground of Personality, not of Substance—in the way of distinction, not of division. But although I must everywhere hold one only substance in three coherent and inseparable (Persons), yet I am bound to acknowledge, from the necessity of the case, that He who issues a command is different from Him who executes it.

Tertullian
Against Praxeas
Chapter XII

The Father and the Son are distinct from each other because they speak to each other

On your side, however, you must make Him out to be a liar, and an impostor, and a tamperer with His word, if, when He was Himself a Son to Himself, He assigned the part of His Son to be played by another, when all the Scriptures attest the clear existence of, and distinction in (the Persons of) the Trinity, and indeed furnish us with our Rule of faith, that He who speaks, and He of whom He speaks, and to whom He speaks, cannot possibly seem to be One and the Same.

Still, in these few quotations the distinction of Persons in the Trinity is clearly set forth. For there is the Spirit Himself who speaks, and the Father to whom He speaks, and the Son of whom He speaks. In the same manner, the other passages also establish each one of several Persons in His special character—addressed as they in some cases are to the Father or to the Son respecting the Son, in other cases to the Son or to the Father concerning the Father, and again in other instances to the (Holy) Spirit.

Tertullian
Against Praxeas
Chapter XI

The Father, Son and Holy Spirit are one and yet three

Bear always in mind that this is the rule of faith which I profess; by it I testify that the Father, and the Son, and the Spirit are inseparable from each other, and so will you know in what sense this is said. Now, observe, my assertion is that the Father is one, and the Son one, and the Spirit one, and that They are distinct from Each Other.

Tertullian
Against Praxeas
Chapter IX

The Logos is numerically distinct from the Father but as a fountain from a river they are also one

“For what man knoweth the things which be in God, but the Spirit which is in Him?” But the Word was formed by the Spirit, and (if I may so express myself) the Spirit is the body of the Word. The Word, therefore, is both always in the Father, as He says, “I am in the Father;” and is always with God, according to what is written, “And the Word was with God;” and never separate from the Father, or other than the Father, since “I and the Father are one.”

…Following, therefore, the form of these analogies, I confess that I call God and His Word—the Father and His Son—two. For the root and the tree are distinctly two things, but correlatively joined; the fountain and the river are also two forms, but indivisible; so likewise the sun and the ray are two forms, but coherent ones. Everything which proceeds from something else must needs be second to that from which it proceeds, without being on that account separated.  Where, however, there is a second, there must be two; and where there is a third, there must be three. Now the Spirit indeed is third from God and the Son; just as the fruit of the tree is third from the root, or as the stream out of the river is third from the fountain, or as the apex of the ray is third from the sun. Nothing, however, is alien from that original source whence it derives its own properties.  In like manner the Trinity, flowing down from the Father through intertwined and connected steps, does not at all disturb the Monarchy, whilst it at the same time guards the state of the Economy.

Tertullian
Against Praxeas
Chapter VIII

The Logos was begotten as God said “Let there be light”

Then, therefore, does the Word also Himself assume His own form and glorious garb, His own sound and vocal utterance, when God says, “Let there be light.” This is the perfect nativity of the Word, when He proceeds forth from God—formed by Him first to devise and think out all things under the name of Wisdom—“The Lord created or formed me as the beginning of His ways;” then afterward begotten, to carry all into effect—“When He prepared the heaven, I was present with Him.” Thus does He make Him equal to Him: for by proceeding from Himself He became His first-begotten Son, because begotten before all things; and His only-begotten also, because alone begotten of God, in a way peculiar to Himself, from the womb of His own heart—even as the Father Himself testifies: “My heart,” says He, “hath emitted my most excellent Word.”

Tertullian
Against Praxeas
Chapter VII