I observe three baptisms or immersions with something and into something: a baptism with confession into repentance, a baptism with water into the name of Jesus, and a baptism with fire into the Holy Ghost. Hence, when John declares the three that bear witness on earth as being the Spirit, the water, and the blood, we can see that at least one of these earthly witnesses corresponds with one of the baptisms. What witness would water have if it is not referring to water baptism? Hence, water baptism is required in order to have water bear witness that we are born of God and have overcome the world (1 John 5:4).
And shouldn't Jesus' declaration to Nicodemus be considered here too, which identifies water and Spirit as necessary elements of the new birth experience? If we do, we can now see two witnesses that correspond with two of the baptisms: water baptism and Spirit baptism, or Holy Ghost baptism. The Spirit/Holy Ghost bears witness of itself when a person is born again of Spirit. The fire that John the Baptist said candidates would be baptized with was identified literally on the day of Pentecost as tongues of flame and it's representation in other instances of Holy Ghost baptisms was tongues. So, for the Spirit to bear witness on earth, it needs to be able to speak for Himself. Tongues are the expected evidence of spiritual birth initially, and the fruit of the Spirit is the expected proof of His residential overcoming power. Which baptism employs the use of the tongue? Is it not the baptism of repentance? With the mouth, confession is made unto salvation! Speaking in tongues (as any tongue talker can testify) is the fire of Holy Ghost emanating from a believer!
The one witness that is remaining to be identified is the blood. When does blood get applied to our salvation experience? What baptism does blood correspond to? How can blood bear witness that we are born of God? I declare unto you that it is when we are baptized into the name of Jesus Christ, for it is His blood that testifies or bears witness to our new birth.
Hence, the salvation process is not complete until all three witnesses have testified.