Versions such as the KJV contain the interpolated Johanneum Comma which is a Trinitarian interpolation that was added in the sixteenth century. Modern Translations such as the NASB and NIV have excluded it because there are NO EARLY GREEK MANUSCRIPTS THAT CONTAIN THIS LATER ADDITION.
The word comma simply means a “short clause,” and Johanneum simply means “pertaining to John.” Without the “comma,” 1 John 5:7-8 reads, “For there are three that testify: the Spirit, the water and the blood; and the three are in agreement.”
With the “comma,” (the added clause) 1 John 5:7-8 reads, “For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.”
Greek Scholar Daniel Wallace published this article in 2004:
“1 John 5:7-8 without the interpolated additions says, “For there are three that testify, 5:8 the Spirit and the water and the blood, and these three are in agreement.” ‑‑NET Bible INTERPOLATED PORTION OF 1 JOHN 5:7 (“For there are three that testify in heaven, the Father, the Word, and the Holy Spirit, and these three are one. 5:8 And there are three that testify on earth”). This reading, the infamous Comma Johanneum, has been known in the English-speaking world through the King James translation. However, the evidence—both external and internal—is decidedly against its authenticity. Our discussion will briefly address the external evidence.”
“This longer reading is found only in eight late manuscripts, four of which have the words in a marginal note. Most of these manuscripts (2318, 221, and [with minor variations] 61, 88, 429, 629, 636, and 918) originate from the 16th century; the earliest manuscript, codex 221 (10th century), includes the reading in a marginal note which was added sometime after the original composition. Thus, there is no sure evidence of this reading in any Greek manuscript until the 1500s; each such reading was apparently composed after Erasmus’ Greek NT was published in 1516.”
“Indeed, the reading appears in no Greek witness of any kind (either manuscript, patristic, or Greek translation of some other version) until AD 1215 (in a Greek translation of the Acts of the Lateran Council, a work originally written in Latin). This is all the more significant, since many a Greek Father would have loved such a reading, for it so succinctly affirms the doctrine of the Trinity.”(Daniel Wallace, https://bible.org/article/textual-problem-1-john-57-8)