Right is about 393.5 years because the iniquity of Solomon's son didn't occur until after the 3rd year but the 390 years are for the iniquity so it would be 390 years.
Let's read together:
2Ki 25:8 And in the fifth month, on the seventh day of the month, which is the nineteenth year of king Nebuchadnezzar king of Babylon, came Nebuzaradan, captain of the guard, a servant of the king of Babylon, unto Jerusalem:
2Ki 25:9 And he burnt the house of the LORD, and the king's house, and all the houses of Jerusalem, and every great man's house burnt he with fire.
2Ki 25:10 And all the army of the Chaldees, that were with the captain of the guard, brake down the walls of Jerusalem round about.
So when was the Temple and Jerusalem destroyed? - Answer - 19th year of Nebuchadnezzar.
And who was the king at the time? - Zedekiah (See earlier verses)
And who was Zedekiah? - he was the last king to serve of the 390 years reigns of the kings of Judah.
Do you see now that the destruction was at the 390th year and that year was the 19th year of Nebuchadnezzar?
Hopefully, you see that now - I mean that sincerely. I want you to get this right.