Amos 1:3

For three transgressions of etc.—As it is stated (Job 33:29) “Lo, all these things does God work, twice, yea, thrice, with a man.” But He does not forgive more (according to Shem Ephraim). And here, it is as follows: I have already behaved toward them according to My standard, to forgive them for three transgressions, and if there are more, surely the smaller number is included in the larger amount.

yea for four I will not return them—For the fourth transgression, I need no longer return him empty- handed from taking his recompense from Me, and surely since there are many transgressions in their hands; now I will visit upon them the retribution for threshing the inhabitants of Gilead, their neighbors.

with sledges of iron—Jonathan renders: morigin of iron. Now that is a sort of file called ‘lime,’ which is made with numerous grooves, and with which they chastise people with tortures, and with them they cut the stubble of the grain to make straw.

1. "Thus said Hashem for the three sins of Damascus..." Amos the prophet was prophecising here about six non Jewish nations. Damascus was the head of the kingdom of Aram, Aza the head of the Phillistine kingdom, Tzur, Edom, Amon and Moab. Why aren't the other nations who also attacked and destroyed Israel mentioned here (eg Babylon, Egypt...). I see two possible reasons. The first is that Amos connected his prophecy to that of Yoel. Yoel's prophecy mentioned the two destructions of the Temple and their judgement ("I will gather the nations and bring them to the valley of judgement and there I will judge them regarding my lot, Israel, that they scattered among the nations and my people they divided" Yoel 4:2) and there it specifically mentions Egypt and Edom becoming a waste. Egypt there refers to the lands of the Ishmaelites who joined the Babylonians in destroying the first temple. Edom refers to Rome who destroyed the second temple. Amos therefore doesn't need to include these nations (Rome? Egypt? Babylon?) in his prophecy about the other nations. He includes nations that Yoel didn't speak of. Yosel does mention Tzur and Sidon and Amos also speaks of Tzur because Tzur was destroyed two times likes it says in Isiah. The first was through Nevudnezer who conquered Tzur and made a great slaughted there. The second conquering of Tzur was through Alexander (the Great?) who destroyed Tzur's army and walls and throughoughly destroyed it that no inhabitants were left there until this day. And Yoel spoke of the final great destruction of Tzur, whereas Amos's prophecy about Tzur referred to the first destruction that would take place through Nevuchadnezer. Also regarding Edom, Yoel said it would become a "desolate wilderness" and Amos also mentioned Edom in his prophecy. Yoel was referring to the Roman Empire and Amos was discusing the sins of Edom connected to Jerusaem and the burning of the south. This is the first reason why the prophet Amos doesn't bring up the other nations who attacked Israel. And the second reason is that the prophet is bring a proof and a reproof for Israel from their wicked neighbors. That if Gd punishes the nations who don't have a connection to Him and don't have the Torah for their sins, how could He also not punish His nation, Israel. And because they accepted the Torah they were more stricken by punishment than the other nations and therefore Amos only brought up examples of close by neighbors to Israel who were punished and not far out island nations (for example Rome). And now I will explain how the punishments are connected to these nations and to Judah together when Israel found themselves in a more serious judgement than the nations and therefore the other sins aren't mentioned only the sins that nations did that are connected to israel.