3 witnesses of Christ here.
Jehovah — the LORD, the eternal I AM
Hear this word that the LORD hath spoken against you, O children of Israel, against the whole family which I brought up from the land of Egypt, saying, Amos 3:1–2
Jehovah chose and knew Israel as His covenant family, so He holds them to a higher standard.
How we know: Jehovah is the premortal Christ (Abraham 1:16; D&C 110:3–4)
Jehovah — the LORD, the eternal I AM
Surely the Lord GOD will do nothing, but he revealeth his secret unto his servants the prophets. Amos 3:7–8
Jehovah always reveals His plans to His prophets before He acts. Prophets testify of Christ because He speaks to them.
How we know: Jehovah is the premortal Christ (Abraham 1:16; D&C 110:3–4)
God of hosts — Lord of the armies of heaven
Hear ye, and testify in the house of Jacob, saith the Lord GOD, the God of hosts, Amos 3:13
Jehovah is named the God of hosts, the commander of heaven's armies, as He calls witnesses against Israel.
How we know: Jehovah is the premortal Christ (Abraham 1:16; D&C 110:3–4)