Hi Wondering,
I would encourage you to seek primary sources for the Early Church Fathers, rather than tertiary sources. What you've posted here is just someone's interpretation of what the writers said. He didn't quote a single writer who said that God is 3 in 1. He believes in the Trinity and sees, the Father, Son, and Holy Spirit, mentioned and assumes it supports his belief. Yeah, he mentioned, the Father, Son, and Holy Spirit. So does the Bible and it doesn't teach that God is three in one. Paul said to the Corinthian Christians, 'to us there is one God, the Father'.
Seriously, reading what someone else thinks the ECF's believed only tells you what that person thinks. I doesn't tell you what the ECF's actually believed. Tertullian did argue that the Father and the Son were two different beings and we see that in Scripture. When the Lord went to Sodom and Gomorrah,
Then the LORD rained upon Sodom and upon Gomorrah brimstone and fire from the LORD out of heaven; (Gen. 19:24 KJV)
Notice what it says. The LORD here is Jehovah, He's on earth and He rains down brimstone and fire from the LORD in Heaven. We have Jehovah on earth and Jehovah in Heaven. That's two Jehovah's. One is on Earth and one in Heaven. This is exactly what Tertullian argued. In this excerpt Tertullian is arguing that the Father and Son are two different beings. This is from Terullian's actual writings
Tertullian, Against Praxeas, Chapter 12
Chapter 12.—Other Quotations from Holy Scripture Adduced in Proof of the Plurality of Persons in the Godhead.
If the number of the Trinity also offends you, as if it were not connected in the simple Unity, I ask you how it is possible for a Being who is merely and absolutely One and Singular, to speak in plural phrase, saying, “Let us make man in our own image, and after our own likeness; ” whereas He ought to have said, “Let me make man in my own image, and after my own likeness,” as being a unique and singular Being? In the following passage, however, “Behold the man is become as one of us,” He is either deceiving or amusing us in speaking plurally, if He is One only and singular. Or was it to the angels that He spoke, as the Jews interpret the passage, because these also acknowledge not the Son? Or was it because He was at once the Father, the Son, and the Spirit, that He spoke to Himself in plural terms, making Himself plural on that very account? Nay, 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 In the following text also He distinguishes among the Persons: “So God created man in His own image; in the image of God created He him.” Why say “image of God? ”Why not “His own image” merely, if He was only one who was the Maker, and if there was not also One in whose image He made man? But there was One in whose image God was making man, that is to say, Christ’s image, who, being one day about to become Man (more surely and more truly so), had already caused the man to be called His image, who was then going to be formed of clay—the image and similitude of the true and perfect Man. But in respect of the previous works of the world what says the Scripture? Its first statement indeed is made, when the Son has not yet appeared: “And God said, Let there be light, and there was light.” Immediately there appears the Word, “that true light, which lighteth man on his coming into the world,” and through Him also came light upon the world. From that moment God willed creation to be effected in the Word, Christ being present and ministering unto Him: and so God created. And God said, “Let there be a firmament, … and God made the firmament; ” and God also said. “Let there be lights (in the firmament); and so God made a greater and a lesser light.” 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. For, indeed, He would not be issuing a command if He were all the while doing the work Himself, while ordering it to be done by the second. But still He did issue the command, although He would not have intended to command Himself if He were only one; or else He must have worked without any command, because He would not have waited to command Himself.
Early Church Fathers - – Ante-Nicene Fathers: The Writings of the Fathers Down To A.D. 325.
I think it's clear from this that Tertullian argued for two separate beings as the Father and Son. Yet, he spoke of the Trinity. As we see though, his idea of a trinity and the modern idea of a trinity are vastly different. The 5th century idea I spoke of is the idea of three coequal coeternal beings who are God. We find that in the Athanasian Creed, not the Nicene Creed and not the Early writers.