This is from the Douay-Rheims Bible, which is in this case the English translation of the original Latin Vulgate of St. Jerome, 4th century.
6 This is he that came by water and blood, Jesus Christ: not by water only, but by water and blood. And it is the Spirit which testifieth, that Christ is the truth. 7 And there are three who give testimony in heaven, the Father, the Word, and the Holy Ghost. And these three are one. 8 And there are three that give testimony on earth: the spirit, and the water, and the blood: and these three are one. 9 If we receive the testimony of men, the testimony of God is greater. For this is the testimony of God, which is greater, because he hath testified of his Son. 10 He that believeth in the Son of God, hath the testimony of God in himself. He that believeth not the Son, maketh him a liar: because he believeth not in the testimony which God hath testified of his Son.
6 “Came by water and blood”… Not only to wash away our sins by the water of baptism, but by his own blood.
8 “The spirit, and the water, and the blood”… As the Father, the Word, and the Holy Ghost, all bear witness to Christ’s divinity; so the spirit, which he yielded up, crying out with a loud voice upon the cross; and the water and blood that issued from his side, bear witness to his humanity, and are one; that is, all agree in one testimony.
10 “He that believeth not the Son”… By refusing to believe the testimonies given by the three divine persons, that Jesus was the Messias, and the true Son of God, by whom eternal life is obtained and promised to all that comply with his doctrine. In him we have also this lively confidence, that we shall obtain whatever we ask, according to his will, when we ask what is for our good, with perseverance, and in the manner we ought. And this we know, and have experience of, by having obtained the petitions that we have made.
As you see, there is no footnote to verse 7, but there are to verses 6 and 8. If the source that had the “trinitarian” reference was the Vulgate, it looks as though it was indeed there, and not added later, IMO.