In my opinion the US would win a war without a doubt considering that our navy is simply too powerful and Russia and China would never be able to have broken a naval blockade.
In a Realistic scenario where there is no surrender, Allies, treaty or peace Russia and china would outlive the U.S.A (Mostly because they have better crowd control + Higher economy) but in the case that there is Allies helping each side U.S.A Has much more western europe support which would massively boost both their land control Russia and china have less allies that are less useful some in eastern europe and a few southern american countries in that case U.S.A (NATO) Would win. Then again i highely doubt this’ll ever happen
The US also depends on Russia for 60% of its energy, meaning it would be at a higher chance to lose the war, also taking into consideration the fact that Russia and China are both communist powers, meaning they don’t have the necessity to appeal to each state or each province in order to start and maintain the war, whereas America could potentially break out into a civil war if on the president was to attempt to control all the states and their trade in order to adequately fight against China and Russia. Also, US is in a lot of financial debt and lack of energy would hit it very hard, even without a war. The US would also receive some Western support, but considering the fact that the majority of the European countries rely on Russia completely for all its energy (German basically can not live without Russia), it would be kind of hard for America to win. In current day circumstances, however, because Russia is having a major defeat with Ukraine (the country is so small, and has such little military power yet Russia is not able to defeat it, even after taking control of the major nuclear plant) America may stand a chance. Also, the irony is in the fact that this question was posted in 2019… and present-day scenarios are very interesting.