Albert Barnes Commentary 2 Samuel 23:18

"And Abishai, the brother of Joab, the son of Zeruiah, was chief of the three. And he lifted up his spear against three hundred and slew them, and had a name among the three." — 2 Samuel 23:18 (ASV)

The three (2 Samuel 23:22). It was Abishai’s prowess on this occasion that raised him to be chief of this triad.