A.T. Robertson Commentary 1 John 5:8

"For there are three who bear witness, the Spirit, and the water, and the blood: and the three agree in one." — 1 John 5:8 (ASV)

The Spirit and the water and the blood (το πνευμα κα το υδωρ κα το αιμα). The same three witnesses of verses 6,7 repeated with the Spirit first.

The three (ο τρεις). The resumptive article.

Agree in one (εις το εν εισιν). "Are for the one thing," to bring us to faith in Jesus as the Incarnate Son of God, the very purpose for which John wrote his Gospel (20:31).