οτι  conjunction
hoti  hot'-ee:  demonstrative, that (sometimes redundant); causative, because -- as concerning that, as though, because (that), for (that), how (that), (in) that, though, why.

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τρεις  adjective - nominative plural masculine
treis  trice:  three -- three.

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εισιν  verb - present indicative - third person
eisi  i-see':  they are -- agree, are, be, dure, is, were.

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οι  definite article - nominative plural masculine
ho  ho:  the definite article; the (sometimes to be supplied, at others omitted, in English idiom) -- the, this, that, one, he, she, it, etc.

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μαρτυρουντες  verb - present active participle - nominative plural masculine
martureo  mar-too-reh'-o:  to be a witness, i.e. testifyNew American Standard Bible
For there are three that testify:

King James Bible
For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.

Holman Christian Standard Bible
For there are three that testify:

International Standard Version
For there are three witnesses —

NET Bible
For there are three that testify,

Aramaic Bible in Plain English
And The Spirit testifies because The Spirit is the truth.

GOD'S WORD® Translation
There are three witnesses:

King James 2000 Bible
For there are three that bear witness in heaven, the Father, the Word, and the Holy Spirit: and these three are one.1 John 5:7
1 John 5:7 NIV
1 John 5:7 NLT
1 John 5:7 ESV
1 John 5:7 NASB
1 John 5:7 KJV