1 John 5:7 in the KJV is spurious. Trinitarians even admit this.
Highly respected trinitarian scholar, minister (Trinity Church), Professor (University of Glasgow and Marburg University), author (The Daily Study Bible Series, etc.), and Bible translator Dr. William Barclay states the following about this passage:
Note on 1 John 5:7
"In the Authorized Version [KJV] there is a verse which we have altogether omitted [in Barclay's NT translation]. It reads, "For there are three that bear record in heaven, the Father, the Word and the Holy Ghost; and these three are one."
"The Revised Version omits this verse, and does not even mention it in the margin, and none of the newer translations includes it. It is quite certain that it does not belong to the original text."
But even if it wasn't spurious, 1 John 5:7 would not mean the three are the one God as trinitarians want. The word for "one" here is in the neuter form, hen, which cannot mean "one God" since "God" is always in the masculine form in NT Greek, and grammatically adjectives (such as "one") applied to it must also be masculine (heis, masculine form). When the neuter "one" (hen) is applied to persons, it means "one thing." In other words they have become united in some thing such as "purpose," "will," etc. (John 17:22)
For much more, see
1 John 5:7 (KJV)