The Spirit and the water and the blood (to pneuma kai to hudōr kai to haima). The same three witnesses of 1Jo 5:6, 1Jo 5:7 repeated with the Spirit first.
The three (hoi treis). The resumptive article.
Agree in one (eis to hen eisin). “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 (Joh 20:31).