Ludicrous scenario and clearly you didn't pass basic military strategy.
China ain't coming to Russia's aid in this scenario as you would end up with 1.4 billion people with no trade markets with the consumer orientated wealthy West and its mineral and energy trade lines between Africa and Iran, severed (as would be it's export shipping market).
Australia wouldn't be shipping any minerals or coal/gas to China and do you think NATO wouldn't interrupt the gas pipelines between China and Russia, if it was at war with them both?
China with its over population and undersupply of energy, minerals and water, is especially vulnerable to a protracted war and it most certainly knows it, even if you don't.
North Korea and Iran?
Neither is set up for a protracted conflict with starving Nth Koreans reliant on China which won't be able to feed itself in this conflict scenario and it's no certainty that the average Iranians are going to die for the mad mullahs, even if you ignore that the Israelis would likely take the opportunity to deal with Iran (with Saudi Arabia silently cheering them on).
More chance with Russia engaged in a conflict with a NATO that is manifestly superior to Russia in military might, that China would seize the opportunity to reverse its ongoing humiliation of old treaties where the Russians were able to take advantage of Chinese weakness to peel away the whole of Outer Manchuria (tip, it was this exact scenario in reverse that gave Russia the opportunity to extort China for Outer Manchuria).
The Ox may be slow, but the earth is patient.
At worst, it would cheer on Poots and take the opportunity to use him as a sacrificial lamb and look to Taiwan.
But won't happen because both Russia and China know, Russia has a snowflakes chance in hell of beating NATO.
