I'm sorry, but this is one of those places where, as they say, rubber meets the road, and we are forced to consider the fact that this particular verse appears in no Greek manuscript before the 14th century.
That's why most translations delete it, and so this verse correctly reads:
Of course, you can decide which to believe, but would ask the question this way: is it easier to believe that Trinitarians added those words to the text beginning in the 1300s, or that anti-Trinitarians somehow deleted them in all previous versions?
1 John 5:7 For three there are bearing testimony in heaven the Father the Word and the Holy Spirit and these three one are
Now if you are one of those who believes that God was unable to preserve His word as it was revealed to the writers of scripture I suppose you could do most anything with what you consider scripture to fit whatever scenario you like. 1 John 5:7 fits perfectly with the rest of scripture and is supported by it.