Albert Barnes Commentary 1 Chronicles 21:12

"either three years of famine; or three months to be consumed before thy foes, while the sword of thine enemies overtaketh thee; or else three days the sword of Jehovah, even pestilence in the land, and the angel of Jehovah destroying throughout all the borders of Israel. Now therefore consider what answer I shall return to him that sent me." — 1 Chronicles 21:12 (ASV)

And the angel of the Lord destroying... These words are not in Samuel, which presents the third alternative briefly. They prepare the way for the angelic appearance in 1 Chronicles 21:16, on which the author is about to lay so much stress.