Walk through Amos, verse by verse, with the great Bible Commentaries of Matthew Henry (1662 - 1714) & Adam Clarke (1760 - 1832)
1:3
Thus saith the LORD; For three transgressions of Damascus, and for four, I will not turn away the punishment thereof; because they have threshed Gilead with threshing instruments of iron: KJV
{The Lord saith these things, On three great trespasses of Damascus, and on four, I shall not convert it, for it threshed Gilead in iron wains. (The Lord saith these things, For the three great trespasses of Damascus, and for the fourth, I shall not turn away their punishment, for they carved, or sliced, up the people of Gilead with iron wagons, or with iron sleds.)} Wycliffe's Bible
Damascus was the capital of Syria.
The God of Israel is the God of the whole earth, and has something to say to them that shall make them tremble. Against them the Lord roars out of Zion. And before God, by the prophet, threatens Israel and Judah, he denounces judgments against those nations whom he made use of as scourges to them for their being so, which might serve for a check to their pride and insolence... Now as to all these nations here arraigned,...
That they are charged in general with three transgressions, and with four, that is, with many transgressions (as by one or two we mean a few, so by three or four we mean many, as in Latin a man that is very happy is said to be terque quarterque beatus—three and four times happy);
or with three and four, that is, with seven transgressions, a number of perfection, intimating that they have filled up the measure of their iniquities, and are ripe for ruin;
That the particular sin which is fastened upon as the fourth, and which alone is specified, is the sin of persecution: