Sunnah.com reference : Book 8, Hadith 128English translation : Book 8, Hadith 0Arabic reference : Book 8, Hadith 1073
A wording by Ahmad has:
Abu Rukana divorced his wife with three pronouncements, in one sitting. Then, he was grieved about her and Allah's Messenger (ﷺ) told him, "They (the three pronouncements) are reckoned as a single (utterance of divorce)."

[The two Ahadith have Ibn Ishaq, in their chains of narrators, and his reliability has been questioned].
وَفِي لَفْظٍ لِأَحْمَدَ : { طَلَّقَ أَبُو رُكَانَةَ اِمْرَأَتَهُ فِي مَجْلِسٍ وَاحِدٍ ثَلَاثًا , فَحَزِنَ عَلَيْهَا , فَقَالَ لَهُ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-" فَإِنَّهَا وَاحِدَةٌ }  وَفِي سَنَدِهَا اِبْنُ إِسْحَاقَ , وَفِيهِ مَقَالٌ  1‏ .‏‏1 ‏- ضعيف .‏ رواه أحمد (165)‏ وليست علته في ابن إسحاق ، وإنما له علة أخرى.‏