In the event of world war 3 between North Korea and USA, which countries will side with North Korea and which with the US?

China are North Korea's ally be that expedient at this stage or not, make no mistake China do not want more US military bases on their doorstep and millions of Korean refugees flooding their country and a possible nuclear fallout in their atmosphere, why people mention Iran in this I'll never know, this is a US fabrication, Iran have zero ties with North Korea.

Since when would a war between NK and the US become a "world war"?

Nobody will "side" with either- they will keep well clear

China has to be on the side of N. Korea only because of the Sino-North Korean Mutual Aid and Cooperation Friendship Treaty (but in reality, China does not really want a war at the moment; it would rather make money and build up its military). Iraq, Myanmar, Venezuela, Cuba, Syria, and possibly, Russia may get in with N. Korea. Additionally, Thailand (a U.S. ally, on paper) may also side with N. Korea only to please China.

S. Korea, Japan, & Taiwan would be backing the U.S. for sure, because they do not like North Korea or China. The U.K. and most of the European countries would be siding with the U.S., if Russia were to be involved. And if the U.K. is involved, then Canada, Australia, New Zealand, and other countries that are in the Commonwealth would be on the side of the U.K. And if Iran and Syria are on the side of N. Korea, then most countries in the Middle East would be on the side of the U.S. And if most countries in the Middle East are sided with the U.S., then the Muslim countries in S.E. Asia such as Indonesia, Malaysia, & Brunei would get on board as well.

India would side with the U.S., but Pakistan would stand idle, because it does not like India.

We'd all be dead 5 months after the outbreak of WW3, so it wouldn't really matter

Everyone who will get involved will side with USA, nobody will side with North Korea unless China or Cuba decide to but that's very unlikely because they know that will destroy them.

everyone with the US,, perhaps iran with NK

Will it really matter.

I think that even China is losing patience with N Korea these days. China is the biggest worry about a larger war breaking out. China doesn't want the USA on their doorstep and that's why they like having N Korea as a buffer but I don't think that they want to blow up the whole world to protect that.

The real problem with N Korea is not the risk of world war, it's the fact that Seoul would have to be sacrificed if there is any military action at all. And it's not even nuclear weapons that is the worry. Seoul, a major world city, is just across the border from N Korea and is in range of conventional artillery. N Korea has thousands of pieces of artillery already in place ready to fire. Even with modern weapon systems it is very difficult to shoot down an artillery shell. That's what makes any military action really difficult.