God goes the extra mile
What about heaven and hell? Spiritual glaucoma? Are not our souls more valuable than 70 years of earth-sight? John is a doctor of these things, and Dr. John reminds us that the voices of the Holy Spirit, the water and the blood are in agreement. Consistent witnesses are more credible. The Hebrew culture demanded two. John says God goes the extra mile. He is telling, reminding his readers of the coherence among not just two, but three witnesses. It is helpful to look at things known and draw proper conclusions from them.
A patient does well to consider the opinions given. A homeowner getting quotes for a home improvement job is wise to ponder the suggestions and costs. Spiritually it is not that different, but just a lot more permanent. Information is available in life, in the scriptures, and via the Holy Spirit. What will we do with it? That determines destiny.