In chapter 3, David gets busy being king and having lots of kids. David has six sons, each by a different wife, and for some reason the retelling of David's family in 1Chronicles 3 gives one name different. It might be a nickname or something like that, but I really have no idea what the story is here.
A really strange story here is the story of Abner and his turning his allegiances to David. Apparently, Abner sleeps with one of Saul's concubines, which worries Ishbosheth (as I think I mentioned elsewhere, it was apparently a custom of the day for a king who conquered another to sleep with the defeated king's concubines to show he is taking the old king's place), and Ishbosheth asks what's going on. Abner flips out and goes over to David's side. (One might wonder if Abner sensed he was on the losing side, and wanted to switch to keep what power he had as a military leader.)
David tells Ishbosheth that he wants his wife Michal back, and Ishbosheth retrieves her. Her new husband Phaltiel is clearly very unhappy about this, but is unable to do anything about it. We're never told whether David did this out of love or out of a sort of display of authority, but we were told earlier that Michal did love David, and perhaps she still does. Something that the SAB does not note here is the fact that a man whose ex-wife has remarried is not allowed to take her back (Deuteronomy 24:4). Since there doesn't seem to have been a formal divorce in this case, it's not clear whether David might be breaking the law in this case.
Now, when David tries to make peace with Abner, Joab hears about it and is very displeased. He goes behind David's back and kills Abner in revenge for the killing of his brother Asahel. David denounces Joab's actions and pronounces a curse on Joab's house. Now whether or not David was right to give this curse (and whether it was really meaningful) is a matter of opinion, but I think I can see David's motivation for being angry with Joab. After all, Abner was offering a chance at peace and a (relatively) bloodless uniting of the kingdom of Israel under David, and Joab messed it up.