Neither Russia nor China would be definitely expected to get engaged in a direct confrontation with the USA. Iran too is not expected to confront the US directly, but it may use its proxies to make life hard in the region to America's allies.
However, errors of judgment on the part of any of these players can occur leading to a much larger confrontation than the envisaged retaliatory bombing of Syrian military installation, which if carried out would start of with missile attacks aimed at, among other, the S-300 anti-aircraft missile installations if, of course, they are already there an operational.
Syria has medium range rockets, which if deployed against Israel could cause a major confrontation in the area envolving also Israel and Iran and even Saudi Arabia.
