'Thirty' in the Bible

David was thirty years old when he became king, and he reigned forty years.

Then three of the thirty chief men went down and came to David at harvest time in the cave of Adullam, while an army of Philistines was encamped in the Valley of Rephaim.

He was honored among the thirty, but he did not attain to the [greatness of the] three. David appointed him over his guard.

Asahel the brother of Joab was one of the thirty; then Elhanan the son of Dodo of Bethlehem,