Jamieson, Fausset and Brown on 2 Samuel 23:19-39

19-39. the first three--The mighty men or champions in David's military staff were divided into three classes--the highest, Jashobeam, Eleazar, and Shammah; the second class, Abishai, Benaiah, and Asahel; and the third class, the thirty, of which Asahel was the chief. There are thirty-one mentioned in the list, including Asahel; and these added to the two superior orders make thirty-seven. Two of them, we know, were already dead; namely, Asahel [2Sa 3:30] and Uriah [2Sa 11:17]; and if the dead, at the drawing up of the list, amounted to seven, then we might suppose a legion of honor, consisting of the definite number thirty, where the vacancies, when they occurred, were replaced by fresh appointments.