In Deuteronomy 19:15 the LORD decrees that only by the testimony of two or three witnesses will a charge be sustained. It is a requirement of the LORD that this be so. Interestingly, a decree from the LORD is established and fulfills this requirement, there are two AND three witnesses, two witnesses in observation, three witnesses in agreement.
When the LORD said, "Let there be light" - it was declared, it was so, and it was established with two witnesses to the member of the Godhead who thus spoke, and with three witnesses in terms of agreement to the truth and validity of the speaking.
Everything the LORD does fulfills this - there is agreement, unity, and truth which is all established in any decree.
Rejection of the idea of the trinity, where the three Persons of God is One, means the falling away of the fulfilling of this requirement.
Check out John 5 where Jesus provides witnesses for his testimony - John the baptist, His own works, the Father, scripture, and Moses... that's 5 witnesses to His testimony. Everything fits! wow...