Albert Barnes Commentary 2 Samuel 23:19

"Was he not most honorable of the three? therefore he was made their captain: howbeit he attained not unto the [first] three." — 2 Samuel 23:19 (ASV)

That is, "Was he not the most honorable of the three of the second order; however, he did not attain to the first three," namely, the triad which consisted of Jashobeam, Eleazar, and Shammah. That two triads are mentioned is a simple fact, although only five names are given.