By numbers, some militaries might appear comparable. China for instance has a huge army, so does North Korea, but in terms of a strike capacity (the offensive ability to bring violence to an opponent) no military in the world is even close to the US military. Neither Russia or China have much of a naval strike capacity. The Russian Army is still a large and modern force with lots of ground weapons. China has a large Army but not as many ground weapons. Both China and Russia are considered to have very good Air Forces. The final necessary component for a strike capability, which is often overlooked, is logistics. Again, no one in the world even comes close to the American's ability to move material and supplies to troops in combat zones.
Iran has a defensive posture, so are only a threat if we attack them. They don't really have a chance in a straight up military fight, but they are a much more capable opponent than Iraq was and are also armed with some of the most advanced anti-ship missiles in the world (courtesy of Russia).
China is rapidly expanding its military and is starting to develop a strike capability, but they aren't ready yet, and they really have nothing to gain and everything to lose by getting into a tussle with America. Yes, they need energy, but they can more easily use their economic clout to get access to energy than they can their military. That still doesn't mean they wouldn't get involved in defending Iran. At some point they will say enough is enough.
Which leaves Russia. It is hard to say what they will do if Iran is attacked, but it is entirely possible they will respond, especially with Putin back in charge. That I think could present a legitimate threat to American forces in the region.