1 Jn 5:7 was correctly translated for centuries in the Latin Vulgate in its Trinitarian Form. I believe the KJV renders it correctly.
Here's the Latin Vulgate with the English Douay Rheims translation:
https://www.drbo.org/drl/chapter/69005.htm
"[7] And there are three who give testimony in heaven, the Father, the Word, and the Holy Ghost. And these three are one.
Quoniam tres sunt, qui testimonium dant in caelo : Pater, Verbum, et Spiritus Sanctus : et hi tres unum sunt."
Here is an Evangelical defense of the Comma Johanneum:
"Appearing first in a late 4th-cent. Latin text, it entered the Vulgate and finally the NT of Erasmus.”1 Other statements along this line abound in liberal and even Neo-Evangelical literature ...Each of these statements, naturally, find much use among Muslim apologists and other antitrinitarians who would probably have little use for anything else contained within these works. "
...
Essentially, the argument Sadler is making is that, because we already “know” that the Comma didn’t appear until centuries after Cyprian, the very clear citation by Cyprian (which, on its merit alone, would be accepted “without the slightest doubt”) must therefore be an “interpolation.” Why? Because that’s what the “accepted” interpretation demands – evidence contrary to the Critical Text dogma of the inauthenticity of the Comma must be explained away and ignored. This sort of argumentation employed by modernistic textual critics is simple intellectual dishonesty. Instead of trying to “explain away” evidence, following Scrivener in accepting Cyprian’s citation of the Comma seems to be the best path to follow. Certainly, our interpretation of later evidences, both pro and contra the Comma, should begin from the virtually certain historical fact (as seen from Tertullian and Cyprian) that some manuscripts in use in the churches around 200-250 AD, whether Old Latin or the Greek from which the Old Latin was derived, had the heavenly witnesses in their texts.
At this point, we should make a comment about the corroborative nature of these witnesses in the Latin. From Tertullian onward, we see several early Latin witnesses to the Johannine Comma. These witnesses, all located in North Africa, do not exist in a vacuum. While Tertullian’s witness from Against Praxeus is less clear, the fact that Cyprian clearly cites the verse, in the same geographical area, a mere five decades later, and makes it significantly more likely that Tertullian did, indeed, have this verse in mind when he used the particular language that he did. So likewise does the testimony of the Treatise on Rebaptism, mentioned earlier, the text of which also dates to this same general time frame and concerns a doctrinal controversy that took place in this specific geographical area.
Cyprian is explicitly corroborated, further, by the fact that Fulgentius, the bishop of Ruspe in North Africa around the turn of the 6th century, both cited the verse in his own writings, and pointedly argued in his treatise against the Arians that Cyprian had specifically cited the Heavenly Witnesses. All of these evidences work together synergistically to shown that the Johannine Comma was recognized in the Latin Bibles of North Africa, both before and after the Vulgate revision was made. Further witness from the 4th century is provided by Idacius Clarus in Spain, who cited it around 350 AD.72"
Taken from:
http://www.verhoevenmarc.be/PDF/Comma-Johanneum-Defence.pdf