John Gill's  Exposition of the Bible 

2 Kings 3:1

Now Jehoram the son of Ahab began to reign over Israel in Samaria the eighteenth year of Jehoshaphat king of Judah, and reigned twelve years.

Now Jehoram the son of Ahab began to reign over Israel in Samaria the eighteenth year of Jehoshaphat king of Judah,.... So that the two years of the reign of his brother Ahaziah were not complete, only part of the seventeenth and part of the eighteenth of Jehoshaphat, since he began to reign in his seventeenth year, at the beginning of that, and died towards the close of the eighteenth, when Jehoram succeeded him, see 1 Kings 22:51, and reigned twelve years.