وَعَلَى ٱلثَّلَـٰثَةِ ٱلَّذِينَ خُلِّفُوا۟ حَتَّىٰٓ إِذَا ضَاقَتْ عَلَيْهِمُ ٱلْأَرْضُ بِمَا رَحُبَتْ وَضَاقَتْ عَلَيْهِمْ أَنفُسُهُمْ وَظَنُّوٓا۟ أَن لَّا مَلْجَأَ مِنَ ٱللَّهِ إِلَّآ إِلَيْهِ ثُمَّ تَابَ عَلَيْهِمْ لِيَتُوبُوٓا۟ ۚ إِنَّ ٱللَّهَ هُوَ ٱلتَّوَّابُ ٱلرَّحِيمُ
“And [Allah has also turned in mercy to] the three who were left behind, [whose guilt distressed them] until the earth, despite its vastness, seemed to close in on them, and their souls were torn in anguish, [until He accepted their repentance so that they could repent].”
Describes Ka'b ibn Malik RA, Murara ibn al-Rabi' RA and Hilal ibn Umayyah RA's anguish during their 50-day boycott before Allah accepted their repentance.
Revealed about the three who were left behind (Ka'b ibn Malik RA, Murara ibn al-Rabi' RA, Hilal ibn Umayyah RA) after their repentance was accepted.
This is the specific, extremely well-documented asbab al-nuzul: Ka'b ibn Malik's own account (quoted in the post) of Allah's mercy after 50 days of being shunned -- the direct occasion for this ayah's revelation.