Although the writer I quote below is desperately grasping at straws to find early proof for the genuineness of the interpolation in 1st John 5:7 he in the process shows proof that the THREE-IN-ONE - ( CONCEPT ) - really originates with Gnostic writers rather than any genuine Christian writer:

CHARLES FORSTER: “...I have myself verified Gk., ( τρία ) in the neuter, applied to ( Persons of the Trinity ), in St. Irenaeus, circ. A.D. 185, St. Hippolytus, A.D. 240, St. Basil, St. Athanasius, Eusebius, and Germanus, Archbishop of Constantinople, A.D. 715. ... The first occurrence of Gk., ( τρία ) I find in St. Irenaeus, in a quotation ( FROM THE ARCH-HERETIC VALENTINUS );{6} a circumstance which carries back its use to A.D. 140, ... from the first verse of St. John's Gospel ; and in this connection so introduces Gk., ( τρία ), ... As he takes HIS GNOSTIC TRINITY from St. John's Gospel, it is only natural that he should take his title for it from the Epistle. We now come to the passage itself: GREEK TEXT: “...Ἔτι [18.] τε [l. δὲ] Ἰωάννην τὸν μαθητὴν τοῦ Κυρίου διδάσκουσι τὴν πρώτην ὀγδοάδα μεμηνυκέναι. αὐταῖς λέξεσι, λέγοντες οὕτως· Ἰωάννης ὁ μαθητὴς τοῦ Κυρίου βουλόμενος εἰπεῖν τὴν τῶν ὅλων γένεσιν, καθ’ ἣν τὰ πάντα προέβαλεν ὁ Πατὴρ, ἀρχήν τινα ὑποτίθεται τὸ πρῶτον γεννηθὲν ὑπὸ τοῦ Θεοῦ, ὃν [ὃ] δὴ καὶ Υἱὸν Μονογενῆ καὶ Θεὸν κέκληκεν, ἐν ᾧ τὰ πάντα ὁ Πατὴρ προέβαλε σπερματικῶς. Ὑπὸ δὲ τούτου φησὶ τὸν Λόγον προβεβλῆσθαι, καὶ ἐν αὐτῷ τὴν ὅλην τῶν Αἰώνων οὐσίαν, ἣν αὐτὸς ὕστερον ἐμόρφωσεν ὁ Λόγος. Ἐπεὶ οὖν περὶ πρώτης γενέσεως λέγει, καλῶς ἀπὸ τῆς ἀρχῆς, τουτέστι τοῦ Θεοῦ καὶ τοῦ Λόγου, τὴν διδασκαλίαν ποιεῖται· λέγει δὲ οὕτως· Ἐν ἄρχῃ ἦν ὁ Λόγος, καὶ ὁ Λόγος ἦν πρὸς τὸν Θεὸν, καὶ Θεὸς ἦν ὁ Λόγος· οὗτος ἦν ἐν ἀρχῇ πρὸς τὸν Θεόν. Πρότερον διαστείλας τὰ τρία, Θεὸν, καὶ Ἀρχὴν, καὶ Λόγον, πάλιν αὐτὰ ἑνοῖ, ἵνα καὶ τὴν προβολὴν ἑκατέρων αὐτῶν δείξῃ, τοῦ τε Υἱοῦ καὶ τοῦ Λόγου, καὶ τὴν πρὸς ἀλλήλους ἅμα, καὶ τὴν πρὸς τὸν Πατέρα ἕνωσιν. Ἐν γὰρ τῷ Πατρὶ, καὶ ἐκ τοῦ Πατρὸς ἡ ἀρχὴ, [ἐν ἀρχῇ δὲ] καὶ ἐκ τῆς ἀρχῆς ὁ Λόγος. Καλῶς οὖν εἶπεν· Ἐν ἀρχῇ ἦν ὁ Λόγος· ἦν γὰρ ἐν τῷ Υἱῷ· καὶ ὁ Λόγος ἦν πρὸς τὸν Θεόν· καὶ γὰρ ἡ ἀρχή· καὶ Θεὸς ἦν ὁ Λόγος, ἀκολούθως· τὸ γὰρ ἐκ Θεοῦ γεννηθὲν, Θεός ἐστιν· οὗτος ἦν ἐν ἀρχῇ πρὸς τὸν Θεόν· ἔδειξε τὴν τῆς προβολῆς τάξιν...” - (Ap. D. Advers. Haeres. Lib. I. p. 36. Op. Paris, 1639. [SAME TEXT = Book 1. Chapter 1. Section 18. (ed. W. Wigan Harvey) Sancti Irenaei episcopi Lugdunensis libri quinque adversus haereses, vol. 1. Cambridge: Cambridge University Press, 1857].)
[ IRENAEUS (circa. 130-200 C.E.): “...And moreover John, the Disciple of our Lord, by their teaching, indicated the first Ogdoad, these being their very words: “John the Disciple of the Lord, meaning to speak of the generation of all things, wherein the Father ( produced them all ), supposes a beginning, the first thing begotten of God, in whom the Father produced all from seed. And by him he saith that the Word was produced, and in him the whole ( substance ) of the Aeons, which the Word himself afterwards reduced to form. Since than he speaks of the first birth, well does he frame his instruction from the beginning, that is, from God, and the Word. And thus he speaks: “In the beginning was the Word, and the Word was with God, and the Word was God; He was in the beginning with God.” Having before distinguished THE THREE, God, and the Beginning, and the Word, he again ( UNITES - THEM ) : to indicate at the same time the ( emanation ) of each of them, – of of the Son and the Word, and their ( UNION ) also with one another and with the Father. For in the Father and of the Father is the Beginning, and of the Beginning is the Word. Well therefore said he, “In the beginning was the Word” ; for he was ( in ) the Son : “and the Word was with God” ; for so was the beginning : “and the Word was God,” by way of inference : for that which is begotten ( of ) God, ( is ) God. “The same was in the Beginning with God,” pointed out the ( order ) of the Production...” - (Page 28 Gnostic interpretation of the Prologue of St. John's Gospel. Valentinian perversion of the Holy Scriptures characterized. Five books of S. Irenaeus bishop of Lyons, against heresies. Translated by the Rev. John Keble, with the fragments that remain of his other works. Published 1872 by J. Parker in Oxford.) ] ... comes from an unexpected quarter. It is given by 'a Greek writer,' not a Father of the Church BUT A HERETIC prior to nearly all the Greek Fathers.{7} In the foregoing example, containing THE GNOSTIC corruption of the doctrine of the Trinity, we have the earliest known instance of Gk., ( τρία ) in the neuter applied to Persons, - to the three Persons of Valentinus's Trinity. The earliness of the date of the document (circa. A.D. 140) precludes the possibility of the ( term ) being borrowed from Christian writers. ( But the idea ) is equally precluded by the circumstances of the case. ... But Valentinus, here, not only takes his Gk., ( τρία ) ... contains the whole idea of the verse – its Trinity in Unity : “...Πρότερον διαστείλας τὰ τρία, [His text omits in this citation: Θεὸν, καὶ Ἀρχὴν, καὶ Λόγον,] – πάλιν αὐτὰ ἑνοῖ...” ... that ValentinusValentinus, we have seen, has been preserved to us by St. Irenaeus...” - (Pages 7-, Chapter 1, “( Τρία ) Used Of The Persons Of The Trinity. “A New Plea for the authenticity of the text of the THREE HEAVENLY WITNESSES: Or Porson's Letters to Travis Eclectically Examined and THE EXTERNAL AND INTERNAL EVIDENCES FOR 1ST JOHN V. 7 ECLECTICALLY RE-SURVEYED.” By Rev. Charles Forster, B.D. Cambridge 1867.)
[PERSONAL FOOTNOTE]: I have added an English version of the Greek in this section from Page 28 of Kebles Translation.

This writer tries to call it a "...corruption of..." the Tri{3}nity doctrine of Christendom. 

Which doesn't wash! 

Because there is no real substantial and un-mistakable evidence of this doctrine WITH-IN CHRISTIANITY in this early period. 

There are only a small few of the "...maybes, if you look at it this way, possibly, sort of, not quite, if you twist it this way..." passages within the ANF. Which is very much against his theory.

But this is evidence of the ALIEN SOURCE of the doctrinal concepts that eventually lead to the corruption of Christianity through false teachers within - as foretold in the Scriptures.