1 Chronicles 3:15 - Albert Barnes' Notes on the Bible

Of the sons of Josiah, Johanan, “the first-born,” who is mentioned in this place only, must, it would seem, have died before his father, or with him at Megiddo; and Shallum (also called Jehoahaz, marginal note and reference) was considerably older than Zedekiah, and was consequently the third, and not the fourth, son. He is perhaps assigned the fourth place here by way of intentional degradation. Compare Jeremiah 22:10-12; Ezekiel 19:3-4.