John Gill Commentary 2 Kings 18:1

"Now it came to pass in the third year of Hoshea son of Elah king of Israel, that Hezekiah the son of Ahaz king of Judah began to reign." — 2 Kings 18:1 (ASV)

Now it came to pass in the third year of Hoshea son of Elah
king of Israel
That is, in the third year of his rebelling against the king of Assyria, when he shook off his yoke, and refused to be tributary to him any longer, see (2 Kings 17:1) ,

[that] Hezekiah the son of Ahaz king of Judah began to reign ;
having finished the account of the kingdom of Israel, and the captivity of the people, the historian returns to the kingdom of Judah, and the things of it.