Kings - Plural because the message Jeremiah 19:3-9, related not especially to the reigning king, but to the whole royal house.
Related Commentaries of Jeremiah 19:3
Jeremiah 19:3
3 And say, Hear ye the word of the LORD, O kings of Judah, and inhabitants of Jerusalem; Thus saith the LORD of hosts, the God of Israel; Behold, I will bring evil upon this place, the which whosoever heareth, his ears shall tingle.