Ether 5:4

And in the mouth of three witnesses shall these things be established; and the testimony of three, and this work, in the which shall be shewn forth the power of God, and also his word, of which the Father, and the Son, and the Holy Ghost beareth record; and all this shall stand as a testimony against the world, at the last day.

Three witnesses will prove these things are true. The three witnesses and this work will show God’s power and his word. The Father, the Son, and the Holy Spirit say this is true. All of this will be used as proof against the world on the last day.

These three witnesses will establish the truth. Their testimony, along with this work that reveals God’s power and word—witnessed by the Father, the Son, and the Holy Spirit—will all stand as evidence against the world on the last day.