וַיְהִי֩ בַיּ֨וֹם הַשְּׁלִישִׁ֜י בִּֽהְיוֹתָ֣ם כֹּֽאֲבִ֗ים וַיִּקְח֣וּ שְׁנֵֽי־בְנֵי־יַ֠עֲקֹב שִׁמְע֨וֹן וְלֵוִ֜י אֲחֵ֤י דִינָה֙ אִ֣ישׁ חַרְבּ֔וֹ וַיָּבֹ֥אוּ עַל־הָעִ֖יר בֶּ֑טַח וַיַּֽהַרְג֖וּ כָּל־זָכָֽר׃
И было на третий день, когда им было больно, двое из сыновей Иакова Симеона и Левия Дина'Братья братья, каждый взял меч свой, и пришел в город врасплох, и убил всех мужчин.
Rashi
שני בני יעקב TWO OF THE SONS OF JACOB — They were his sons (the sons of Jacob, a wise and clever man), but they acted as any man named Simeon and Levi would do — as other people would do who were not his sons — for they did not take counsel with him (Genesis Rabbah 80:10).
Rashbam
בהיותם כואבים, according to normal reactions of people to such an operation, and according to the plain meaning of the text, the meaning is that they had been in pain during the first and second day after the circumcision, and they were now in even greater pain,
Radak
ויהי ביום השלישי בהיותם כואבים, this was because the pains are strongest on the third day after the circumcision. In view of this the sages permitted bathing the child with hot water on the Sabbath after the circumcision, if it coincided with being the third day after the operation.(Shabbat 134). Even hot water boiled on the Sabbath itself is permissible to be used for this when there is no other, as the potential danger to the life of the infant overrides the Sabbath legislation not to boil anything on that day.