Everything belongs in its proper place. The Gospel is followed by water baptism which is followed by the Lord's Supper. No one should assume that the Gospel nullifies the rest of Christ's teachings.Verse 3 does not say anything about water baptism. repenting or the requirement to eat a ritual Lords Supper.
As to repenting, it is built into the Gospel: And [Christ] said unto them, Thus it is written, and thus it behoved Christ to suffer, and to rise from the dead the third day:And that repentance and remission of sins should be preached in his name among all nations, beginning at Jerusalem. And ye are witnesses of these things. (Luke 24:46-48)