Taken from commentary of shia scholar Ayashi, narration #148:

From Khannan ibn Sadeer from his father from Abu Jafar (alaihi salam): “People became apostates after (death of) prophet (sallalahu alaihi wa ala alihi wa sallam)”. And I said: Who are these three?”. He said: “al-Miqdad, Abu Dharr and Salman al-Farese”…

In “Kitab Sulaym ibn Qays” written:

Ali (alayhi salam) said: “All people apostated after prophet (sallalahu alaihi wa ala alihi wa sallam) except 4 of them”.

Al-Kulayni says in Rawdhatul Kaafi Vol.8  pg. 235, and Majlisi in “Mirat” said it’s hasan or muwathaq:

‘People became apostates after the death of the Prophet (Sallallaahu Alayhi Wasallam), except for three people: Miqdad ibn Aswad, Abu Dharr Ghifari and Salmaan Farsi.’