Actually for China and Russia, the best scenario is not a quick defeat of the US against Iran. Cause then US might refocus against them and give up on the middle east.It’s clear the U.S. and western democracies have an Achilles heel and that is high casualties. They would not be able to sustain Ukraine level or battle of the Somme type casualties and remain in power.
If China or Russia really wanted to stick it to the Americans, enable Iranians to taken down and sink a U.S. carrier and show livestream drone footage of struggling, dying and dead servicemen.
Best case is a long quagmire of many years being stuck in the middle east wasting away trillions.