Why does David have or what’s the difference of “3” “30” and “37” mighty men?
There seem to have been a certain three mighty men who were the greatest of all of them. These are listed as Josheb-Basshebeth the Tachmonite, Eleazar the son of Dodo, and Shammah the son of Agee the Hararite in II Samuel 23:8-12. These we might call the “top three.” Then, there was a second three. The first two of these are Abishai the son of Zeruiah and Benaiah the son of Jehoiada in II Samuel 23:18-23. The third of this second three was probably Joab, though he is not listed due to his final disloyalty to David when he followed Adonijah.
After these two sets of three there was a group called the “thirty mighty men.” These were not as great as the first two sets of three, but they were great enough to be considered “mighty men” over just regular officers in the army. Their number, even in II Samuel 23, does not seem to have been exactly thirty, as thirty-one names are listed, even once one subtracts the first two “threes.” Yet this name was probably a name they came to be known by, and once it caught on, it remained, even when their numbers swelled to larger than thirty. This is true in college football, for example, wherein the “Big 10” has twelve teams, the “Big 12” has ten teams, and the “Pac 10” has twelve teams. Or something like that. Who can keep track? Anyway, the name “thirty mighty men” stuck, even when they had more members. In I Chronicles 11, the number had expanded by at least sixteen more members, though they are not actually called the “thirty” in this case.
The “37” number is mentioned in II Samuel 23:39, attained by counting up all the names in that chapter, and then adding Joab. It includes the first three, the second three, and the “thirty” (actually 31.)