They only will talk about economy not other issues because that can avoid complicated issues with nearby other countries. And this all 3 parties will get their best interests as big win win.
If that meeting becomes successful,it can lead many stuffs like the World economy boom and Asia's peace and eventually Noth Korea does not have to want to keep any nuclear weapons.
This success can delete the nuclear weapons fastest way.
I don't think China or Russia will lose any thing either. If that part of Asia will be good economic success zone. China will be happy,Russia will be happy because South Korea will invest more money in Russia and there will be big one train from South Korea to Moscow and to the paris to London too.
China and Russia can invest their money to the North Korea too.