Albert Barnes Commentary 2 Kings 3:1

"Now Jehoram the son of Ahab began to reign over Israel in Samaria in the eighteenth year of Jehoshaphat king of Judah, and reigned twelve years." — 2 Kings 3:1 (ASV)

In the eighteenth year of Jehoshaphat - This date agrees exactly with the statements that Jehoshaphat began to reign in the fourth year of Ahab (1 Kings 22:41), and Ahaziah in the seventeenth year of Jehoshaphat (1 Kings 22:51).