Charles Ellicott Commentary 2 Samuel 23:24

"Asahel the brother of Joab was one of the thirty; Elhanan the son of Dodo of Beth-lehem," — 2 Samuel 23:24 (ASV)

Asahel. —As he was killed by Abner while David reigned over Judah only, it is plain that this list is not restricted to any one definite time in David’s reign. Leaving out Asahel, however, the names that follow are exactly “thirty.” Anything further is known of only a few of them.