A third part - This seems to have been a favorite division with the Hebrew commanders (see Judges 7:16; Judges 9:43; 1 Samuel 11:11; 2 Kings 11:5-6) and with the Philistines also 1 Samuel 13:17.
Related Commentaries of 2 Samuel 18:2
2 Samuel 18:2
2 And David sent forth a third part of the people under the hand of Joab, and a third part under the hand of Abishai the son of Zeruiah, Joab's brother, and a third part under the hand of Ittai the Gittite. And the king said unto the people, I will surely go forth with you myself also.