1 John 5:7

three--Two or three witnesses were required by law to constitute adequate testimony. The only Greek manuscripts in any form which support the words, "in heaven, the Father, the Word, and the Holy Ghost, and these three are one; and there are three that bear witness in earth," are the Montfortianus of Dublin, copied evidently from the modern Latin Vulgate; the Ravianus, copied from the Complutensian Polyglot; a manuscript at Naples, with the words added in the Margin by a recent hand; Ottobonianus, 298, of the fifteenth century, the Greek of which is a mere translation of the accompanying Latin. All the old versions omit the words. The oldest manuscripts of the Vulgate omit them: the earliest Vulgate manuscript which has them being Wizanburgensis, 99, of the eighth century. A scholium quoted in MatthÃ&brvbri, shows that the words did not arise from fraud; for in the words, in all Greek manuscripts "there are three that bear record," as the Scholiast notices, the word "three" is masculine, because the three things (the Spirit, the water, and the blood) are SYMBOLS OF THE TRINITY. To this CYPRIAN, 196, also refers, "Of the Father, Son and Holy Spirit, it is written, 'And these three are one' (a unity)." There must be some mystical truth implied in using "three" (Greek) in the masculine, though the antecedents, "Spirit, water, and blood," are neuter.

— Jamieson-Fausset-Brown Commentary (public domain)