If N. Korea decided to use nuclear weapons they would be decimated not only by the US, S. Korea, and Japan, but also China. The last thing China wants is nuclear weapons being used close to its borders.
This is not to say that there wouldn't be casualties. It would not be a war won over night. However, it would be a conventional war that N. Korea would be unable to win.
Somebody posted earlier about N. Korea not actually wanting to use nuclear weapons, but sell them. That is a more likely scenario that still leads to the above happening. A conventional war that will, in time, lead to the downfall of N. Korea.