“For the Lord GOD does nothing without revealing his secret to his servants the prophets.
But he had before spoken of the sound of the trumpet; for every excuse was thereby taken away from the Israelites, as God had not only recalled them to the right way by his scourges but also preceded these by his word: and he shows how justly he was displeased with them; hence the Prophet adds another sentence, For the Lord Jehovah will do nothing...
The scope of these verses is to convince the people of Israel that God had a controversy with them. That which the prophet has to say to them is to let them know that the Lord has something to say against them, Amo 3:1.
Commenting on Amos 3:1-8
The lion hath roared, who will not fear?.... Amos said this from his own experience, who, having been a herdsman in the wilderness of Tekoa, had often heard a lion roar, which had put him into a panic, both for himself, and the cattle he kept; the figure is explained in the next clause: the Lord God hath spoken, who can but prophesy?