Epi check out the greek.......
For there are three that bear witness, the Spirit and the water and the blood; and the three are in agreement.
agreement (in this verse)Greek Word: Ei|ß
Transliterated Word: heis
definition: ONE
If you want some verses that are super super old
try genesis
Ge 1:26 Then God said, "Let Us make man in Our image, according to Our likeness; and let them rule over the fish of the sea and over the birds of the sky and over the cattle and over all the earth, and over every creeping thing that creeps on the earth."
Ge 1:27 And God created man in His own image, in the image of God He created him; male and female He created them.
It goes from plural to singular......One definition of Eloyhim(God) which is the hebrew used in these verses is (plural intensive - singular meaning) GOD