That one is found only in the King James Version of the Bible, and it's universally recognized to have been added in the middle ages. Most Christians in the US use more modern versions of the Bible, like the NIV or RSV, which don't have that "these three are one" thing.
Firstly - You are ignoring all the other verses from John.
Secondly - That is not the impression I get.... EVERY SINGLE TIME I talk to a christian they DEMAND that any quotation has to be from the KJV or they would not even begin to look at it.
Actually most, after I show them the KJV and they are cornered, resort to the demand of the original Greek and Hebrew despite actually not being able to read or understand MODERN Greek or Hebrew let alone ANCIENT Greek and Hebrew in which the bible was written and lost and rewritten numerous times.
Thirdly - Have a look at the verses and see that in fact they all imply a TRINITY…. They just give them different names. They also seem to imply that Jesus is AMALGAMATED as part of the three.
Fourthly - if you go by the others it seems to me PAGANISM and that there are THREE GODS. So in fact if you go by the others you then have to admit that Christianity is a PAGAN POLYTHEISTIC religion.
Fifthly - What NONSENSE is the whole thing…. What has water and blood got to do with testifying? Can water speak? Can blood testify?
So I think the meaning here is quite clear…. Jesus who is the water and blood and OF the spirit is also one of the trinity…. Spirit, Jesus, God.
Here are the verses… read them and compare.
KJV 1 John 5
6 This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth. 7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. 8 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.
NIV 1 John 5
6 This is the one who came by water and blood—Jesus Christ. He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth. 7 For there are three that testify: 8 the[a] Spirit, the water and the blood; and the three are in agreement.
NRSB 1 John 5
6 This is the one who came by water and blood, Jesus Christ, not with the water only, but with the water the blood. And the Spirit is the one that testifies, for the Spirit is the truth. 7 There are three that testify: 8 the Spirit and the water and the blood, and the three agree.
NASB 1 John 5
This is the One who came by water and blood, Jesus Christ; not with the water only, but with the water and with the blood. It is the Spirit who testifies, because the Spirit is the truth. 7 For there are three that testify: 8 the Spirit and the water and the blood; and the three are in agreement