2 Samuel 18:2 (BSB)

He sent out the troops, a third under Joab, a third under Joab’s brother Abishai son of Zeruiah, and a third under Ittai the Gittite. And the king said to the troops, “I will surely march out with you as well.”

From 2 Samuel 18. Also in the ESV.

Commentary on 2 Samuel 18:2

• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on 2 Samuel 18:1-8: Which way David raised an army here, and what reinforcements were sent him, we are not told; many, it is likely, from all the coasts of Israel, at least from the neighbouring tribes, came in to his assistance, so that, by degrees, he was able to make head against Absalom, as Ahithophel foresaw. Now here we have, I. His army numbered and marshalled, Sa2 18:1, Sa2 18:2.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on 2 Samuel 18:2: And David sent forth a third part of the people under the hand of Joab,.... Very likely that which made the centre of the army, since Joab was the general of the army; though this distribution was made when David thought to have headed the army himself, and so made with respect to that: and a third part under the hand of Abishai the son...
• Keil & Delitzsch (Lutheran), Biblical Commentary on the Old Testament on 2 Samuel 18:1-3: 2Sa 18:1-3 Preparation for war. - 2Sa 18:1-2. David mustered the people that were with him, and placed over them captains of thousands and hundreds, and divided them into three companies, under the generals Joab, Abishai, and Ittai the Gathite, who had given such decided proofs, according to 2Sa 15:21-22, of his fidelity to David.