Russia survives; business as usual, if much poorer. China doesn't want to poison that relationship.
Russia falls; China helpfully "adopts" the orphaned Asian lands.
Iran falls; turmoil follows; the USA as usual (since WWII) has no plans for afterwards. Do nothing until opportunity presents itself.
Iran survives; the US falters; wait and benefit from the opening that creates.
I can't see a path where China picking sides in UKR/RUS nor USA/IRAN benefits China at all.