If world war three will happen on this coming end of the year maybe those country that support us who bomb recently Syria and Afghanistan will be declared a war those who are not supported like china and Russia. They will like killing those innocent people around them.
At the moment I see no reason why China entered the war. Where America is opposed to the interests of China? Only in Economics, but this is an economic war and it is a long time and never stops. Russia is very weak state to fight with America. Russian can only secretly support the enemies of America and Europe. In open confrontation they will not go.
China doesn't want any war specially with USA. They will war with every nation but I doubt it will go and war with Uncle Sam. China is a bully, look at how they bully small nation like Philippines regarding the claims of the West Philippine Sea. They are also claims some of the island and established military stations. But USA is another thing, they can't bully another bully. And same as US as well. They will rather go to war with NoKor or in the Middle East.