US-China has annual trade of 500 billion, plus China has about 3 Trillion in American bonds. If at all a conflict breaks out between these two, even before the first bullet is fired it will be an economic war resulting in a total global economic meltdown. It's unlikely that these two will ever indulge in the kind of 'wars of choice' that these both have indulged with smaller nations in the past.
At the most America can fight a defensive war with China.
As far as American support to India, think about why the US hates China? has China ever fired a single bullet at the US? has any Chinese citizen ever burnt an American flag calling for 'death to America'? Why is it then that the US hates it? that's because the US doesn't want any Asian giant to surpass American hegemony. In that case why would they support India? how is India any different than China in the American strategic calculations? So if they are supporting us, the scope will be limited to counterbalance China and will not be any drastic assistance which will increase our actual power. They don't love India any more than China. They want to contain both. It would be naive on our part to assume that the US wants us to expand our strategic capabilities.
India, US, Russia, China are a part of the multipolar world where each independent pole is trying to impede the other. Sometimes with each other's help, sometimes alone. There can never be an alliance between these powers. We can't rely on any foreign nation for help during war. All we can do is use the temporary peace to build ourselves up.