Amos 1:11

for pursuing their brother with a sword—As it is said: (Num. 20:18) “Lest I go forth against you with the sword.” And he still clings to his anger, keeps his wrath, and has not retracted it.

and grasped forever their anger—He clung to it and did not let it go.

The prophet then goes on to decree regarding Edom. I already explain the meaning of the three sins of Edom. The first was that in the days of Esau, he pursued Jacob his brother. The second sin was in the days of Moshe Rabbeinu when the Israelites sought to pass in Edom's land they didn't allow them to pass through and threatened (Bamidbar 20:18) lest a sword come to greet you. The third sin occured during the destruction of the first Temple (Psalsm 137:10) as it says "Gd remembers regarding the sons of Edom, the day of Jerusalem that they said Aru, aru" and the fourth sin was that the Edomite descendants, who were the Romans destroyed the second Temple and spilled the blood of the Judeans like water around Jerusalem. And the prophet Amos says about this "for the three sins of Edom..." The explanation is that the prophet says "he pursued his brother with a sword" this refers to the first sin of Esau pursuing Jacob to do evil to him because even though he wasn't able to harm him, his intention was murder. The second sin is referenced by Amos's statement "he curbed his mercy". The third sin is referenced by "his anger raged unceasing" and refers to the destruction of the first temple and possibly here the word "l'ad" or "unceasing/forever" comes from the same root as in (Breishis 49:27) where the word "ad" means the spoils of the war, and during the destruction of the first temple, Edom came to Jerusalem to take the spoils. And regarding the destruction of the second Temple and the extended exile and the all the tragedies that Judah has borne from the nation of Edom, the prophet says "his fury lasted forever" and the word "netzach/forever" references the length of the exile. But, I also gave a different explanation that can be correct where the three sins that are overlooked refer to the three more serious trangressions which are idolatry, immorality and murder and the fourth references their persecution of Israel. In which case they were certainly deserving of punishment for the first three sins alone but were only punished for the fourth. And the fourth sin is mentioned in the pesukim. The prophet says "he pursued his brother with the sword" this was when Israel passed them in the desert and Edom came out to meet them and Moshe Rabbenu sent them word of peace "thus said your brother Israel" and Edom in their cruelty, curbed their compassion and their brotherly feelings and chase them with the sword. And they continued to display rage and animosity towards the nation of Israel that lasted eternally even after Israel settle in their land Edom continually acted as an enemy. And therefore, Gd decreed that midah k'neged midah, deservingly, since their animosity towards Israel was everlasting, Gd would also harbor animosity towards them eternally.