OET interlinear 1 YHN (1 JHN) 5:8

5:7-8 three witnesses: The Spirit descended on Christ at his baptism (see John 1:32-34). The water is the water in which Christ was baptized (see Matt 3:13-15; Mark 1:9-11). The blood is the blood that Christ shed at his crucifixion (see Mark 15:37-39). All three proclaim Jesus as God’s Son (1 Jn 5:6).
• After the phrase three witnesses, a few very late manuscripts add in heaven—the Father, the Word, and the Holy Spirit, and these three are one. And we have three witnesses on earth. The longer version was written in Latin several centuries after John to explain the three elements (water, blood, and Spirit) as symbols of the Trinity. This explanation found its way into some Latin editions of 1 John, including later copies of the Latin Vulgate. Eventually, Erasmus translated it into Greek and included it in what became the Textus Receptus, the “received text,” which is why it was included in the King James Version. The longer version cannot be found in any Greek manuscript prior to the 1700s and was never cited by any of the early fathers of the church. For these reasons, few modern English translations recognize the longer version as part of the authentic text.