If there is a war between Iran and the US (and I am not even talking about Israel here because the kind of fire power that the US alone can bring to bear on Iran will make Israel look like a novice in comparison), neither Russia nor China will get involved. As simple as that, moral and diplomatic support yes, but to get out there and get involved militarily, no way. Mark my words. Yes Russia will scream bloody murder, China will express its extreme anguish, guys like Hugo Chavez will rant and rave but no one will send one lousy soldier into the battle. India will be somewhere there with these countries, asking for the war to be stopped.
Now if Israel attacked and the US didn't then it will be a different ball game. Even then China and Russia will not get involved outright in the war. But a lot of stuff will start moving into Iran through the north, through the Caucasus, Black Sea and Turkmenistan. Turkey will take a confrontational stand and its air force and navy will be active against Israel. Many other west Asian countries will get involved. Pakistan? Pakistan will give all support to Iran but stop short of declaring a war on Israel.
Personally, I do not believe that Israel can win a war against Iran alone. Israel will therefore use all means at its disposal to get the US involved. It will be a very sad day if war breaks out.