Albert Barnes Commentary 2 Samuel 18:2

"And David sent forth the people, a third part 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." — 2 Samuel 18:2 (ASV)

A third part - This seems to have been a favorite division with the Hebrew commanders (Judges 9:43; 1 Samuel 11:11; 2 Kings 11:5–6) and also with the Philistines (1 Samuel 13:17).