v1: The events of chapter 2 continued for two years, as David grew stronger.
v2-5: David had several wives, which set a bad example for Solomon, see Deut 17.14-17. It is suggested that David's marriage to Maacah, daughter of the king of Geshur, was to promote good political relations; in contrast, Solomon loved many foreign women (1 Kings 11.1-2). Of the six sons born at Hebron, three caused problems later, Amnon, Absalom, and Adonijah.
v6-11: Abner's disagreement with Ishbosheth. Abner had given Ishbosheth his power, but at the same time, he strengthened his own position. Abner expected Ishbosheth to ignore his transgression, as a mark of gratitude for his loyalty.
Abner recognised that he was supporting a defeated kingdom, since the Lord God had promised the throne to David. Thus Ishbosheth's final defeat was only a matter of time. Abner also knew that he could transfer the whole kingdom over to David.
v12-14: Abner took the opportunity to send a message to David. David also knew that Abner was the key to winning over the whole of Israel. David accepted Abner's offer, but only on the condition that Michal, who had been married to David, would be returned to him. David had married Michal (1 Sam 18.20-29), but after David had fled from Saul (1 Sam 19.17), Michal had been given to Paltiel (1 Sam 25.44). Later, we find that Michal did not share David's spiritual desire (6.16); David may have been wiser to have forsaken his 'right' over her.
v15-16: David's message about Michal was sent to Ishbosheth, and he responded.
v17-19: Abner approached the elders of Israel. He used the promise that David would deliver them from the Philistines, and from all their enemies. This would have been effective, since Ishbosheth was impotent.
v20-21: David accepted Abner; his attitude contrasted with Joab's suspicion. But Abner had killed Asahel, Joab's brother, so he had a 'reason' to feel as he did. Joab was not spiritual, but he was loyal. It may be that David should have identified these attitudes, and brought Joab and Abner together in peace.
v22-23: As Abner was leaving David, Joab returned. He was told of Abner's visit, and either did not hear the whole story, or he may not have accepted what he was told.
v24-25: Joab accused Abner of spying.
v26-27: Joab sent a message to bring Abner back, but did not tell David. This presented him the opportunity to kill Abner.
v28-30: David pronounced a curse upon Joab's family. Joab had used deceit to kill Abner.
v31-35: David had genuine sorrow over Abner, as he had for Saul and, later, for Ishbosheth. He had no desire to 'get even', nor to criticise the one who could not answer back. David even commanded Joab to mourn with him, and Joab doubtless heard David's lament, referring to Joab as "wicked". Later (19.5-8), Joab criticised this attitude in David.
v36-39: The people knew that David had not wanted Abner killed. He described Abner as "a prince and a great man". In contrast Joab was "too strong" (or harsh), and an evildoer.