In the first chapter Amos is difficult and obscure, where he speaks of three and four sins. Many have knocked themselves out over it, struggling with it at great length. But the text, I believe, shows clearly that these three and four sins are but one sin. For he always names and cites only one sin. Against Damascus, for example, he names only the sin that "they have threshed Gilead with iron chariots," etc. [1:3].
But Amos calls this sin "three and four" because the people do not repent of the sin or acknowledge it; rather they boast of it and rely upon it as though it were a good deed, as false saints always do. For a sin cannot become more grave, great, or weighty than when it tries to be a holy and godly work, making the devil God and God the devil. So, too, three and four make seven, which is the completeness of numbers in the Scriptures, where one turns back and begins to count again, both the days and the weeks.
Martin Luther on Amos.
(Intro: Lutheran Study Bible, p. 1456)