Albert Barnes Commentary 1 Chronicles 3:15

"And the sons of Josiah: the first-born Johanan, the second Jehoiakim, the third Zedekiah, the fourth Shallum." — 1 Chronicles 3:15 (ASV)

Of Josiah's sons, Johanan, the first-born, is mentioned only here and seems to have died either before his father or with him at Megiddo. Shallum (also called Jehoahaz) was considerably older than Zedekiah and was therefore the third son, not the fourth. He is perhaps assigned the fourth place here as an intentional degradation. Compare Jeremiah 22:10-12; Ezekiel 19:3–4.