“three sons.” They apparently all died young (see commentary on 2 Sam. 18:18).
“Tamar.” A name in David’s family with a long history (see commentary on 2 Samuel 13:1). It is most likely that Tamar was also called “Maacah” (1 Kings 15:2; 2 Chron. 11:20), and she married Rehoboam the son of Solomon and gave birth to Abijah, the second king of the southern kingdom of Judah (2 Chron. 11:20-22). It would not have been uncommon for someone in the royal family to have a second name, and especially so since her living aunt was also named Tamar. The Septuagint translators thought so, and added this to the Hebrew text: “And she became a wife to Rehoboam the son of Solomon and to him she gave birth to him Abia [Abijah].” Thus Absalom, who so badly wanted to be king, never was, but his grandson became king of Judah (see commentary on 1 Kings 15:10).
It has been suggested, but it is much less likely, that the Maacah that Rehoboam married was the daughter of a different Absalom, but there is no other Absalom in the Bible and no reason another would be introduced into the narrative at this point without some kind of clarification.
“a woman who was beautiful in appearance.” Abraham used the same phrase when speaking of his wife Sarah (Gen. 12:11). Absalom’s sister Tamar was beautiful (2 Sam. 13:1), and his daughter, whom he named Tamar, was also beautiful. David himself was handsome (1 Sam. 16:12), and since royal wives were usually beautiful (cp. Esther 2:2-4), it makes sense that the women in royal families were usually beautiful.