The words you seem to be referring to appear for the first time in 1215.
See:
The words “in heaven: the Father, the word and the Holy Spirit, and these three are one. And there are three that testify on earth” are usually said to be found in only four Greek MSS (61 88mg 629 635ms—so Metzger, 717; but UBS gives 61 88mg 429mg 629 636mg 918). None of these is earlier than the fourteenth century. The passage is quoted by none of the Greek church writers, and it first appears in Greek in a council report of 1215. None of the ancient versions of the New Testament contains the words, except the Latin version. The words appear, with considerable variation (including inversion of the order of the two sets of witnesses), in some Old Latin and Vulgate MSS, but not in the earliest form of the Old Latin or in Jerome’s edition of the Vulgate. They are attested by a number of Latin writers, the earliest certain reference being in the Liber Apologeticus of the Spanish writer Priscillian (ob. c. 385) or his follower Instantius. It is wholly improbable that such a weakly attested reading is an original part of the text of 1 John, and the added words cause a break in the sense. The addition appears to rest on allegorical exegesis of the three witnesses in the text; it was probably written in the margin of a Latin MS and then found its way into the text; later still the order of the two sets of witnesses was inverted and the text was translated back into Greek and was included in a few Greek MSS. Erasmus rejected it from the first two editions of his Greek New Testament; he said that he would include it if a single Greek MS could be produced containing the words. Such a MS (61), probably written in 1520, was produced, and Erasmus had to keep his word in his third edition (1522), although he protested forcibly; subsequently, he again omitted the words. But the words remained in the Vulgate, and modern Roman Catholic translations, based on the Vulgate rather than the Greek text, included them (so Knox); the most recent Roman Catholic translations (such as the Jerusalem Bible) omit them. There remains some doubt as to how far back the variant reading can be traced in the Latin tradition (Schnackenburg, 46; W. Thiele, “Beobachtungen zum Comma Johanneum (1 Joh. 5, 7f.),” ZNW 50, 1959, 61–73), but this does not affect the basic issue. Elsewhere the Latin text of 1 John shows other interpolations and alterations (2:17; 4:3; 5:6, 20).
I. Howard Marshall, The Epistles of John, The New International Commentary on the New Testament (Grand Rapids, MI: Wm. B. Eerdmans Publishing Co., 1978).