Yes, their militaries are inferior.
They are near peers. Every expert agrees. If we were to manage to defeat them, Russia specifically, it would rain nuclear weapons down on the US and we would not exist as a functional nation any longer. Russia has more, and far more advance, nuclear weapons than the US. We would be unable to stop such an attack.
Yes, they could harm us both militarily and domestically.
Economic attacks would collapse our currency. How would we fund anything? Grid attacks would shut down our power grids? How would we power anything? We import nearly everything to run our nation. Those ships and planes would be sunk and shot down until no more come. How would any of our industries like trade and travel, survive? In 1 year the US would be collapsed from within thru military and unconventional warfare. Neither China nor Russia are as vulnerable for a host of reasons.
Military fights would be on the order of WWI and WWII bloodshed. The US has never lost as many in any war as it would lose in such a multi-front war. Tens of millions of men and women would not come home. Do you honestly think the typical American has this resolve today? I don't think Russia or China would flinch, losing 10 million men.
The reason these conflicts were lost was because of the restraint shown by our military forces. My Dad served in the 8th Air Force during WWII, based out of Snetterton-Heath. To defeat the enemy requires total war on the civilian population. This is why WWII was won by American and its allies (and Russia too wiith respect to Germany). It requires annihilating the civilian population. Is this something I want? No. But it is the way wars have been won in the past.
Maybe, maybe not. Since the 50s or so, due to international agreements, the US generally tries in modern wars to not target civilian infrastructure and/or populations. I suspect we would, at least initially, abide by those rules of armed conflict. (FYI I was an instructor on such rules in my Active Duty days.) It would be interesting to see if our enemies adhere to such wartime "rules." We see that Russia is bombing civilian areas in Ukraine, but partly because the military is illegally using civilian areas as shields. Hard to say, but I suspect Russia and China would have no qualms of targeting civilian infrastructure. Would the US retaliate in kind, not unless it became legal because it is not legal now, so that is an unknown.
But could they persevere and actually win? I don't think so.
What would "winning" or "victory" look like? Whether conventional, unconventional, economic, or I shutter to think nuclear war with either nation, I believe most of us would probably die or suffer great loses. Keep in mind, insurance does not tend to cover property damaged by war or terrorism. So, imagine you lose all your assets in a bombing and reset your finances to zero. Or hacks to the markets erase all your digital assets. Or many of your close friends, relatives, neighbors, are victims of war. Starvation, Energy crisis, deep economic hardships for Americans, possibly entire big cities just destroyed... these are all very plausible outcomes that folks never factor. Folks never think they'll be impacted, it's just some far away war on TV... Modern tech and weapons can reach you at your zip code today.
Unlikely war sabotage, but just look at a few instances today - various bird and mammal viruses destroying livestock, train derailments like in Ohio, mass shootings, people hitting electric grids, etc. all causing real hardships.
Deaths from WWI and the Spanish flu, over a period of about 4 years, saw something like 100,000,000 deaths representing about 10% of the globes population, with far less effective warfare weapons (they had no city leveling weapons as we do) limited in scope, size, geography, and range. Almost nobody in the US was killed, for instance. A comparable number today with 8 billion people, would be 10% of that, or 800 million. In reality it would probably be a higher % of deaths given the size, range, sophistication of weapons, and our reliance on energy grids (e.g. shut the grids off on the eastern US seaboard for 2 weeks and millions would die). So think of everyone you know and consider that a modern war would likely yield higher numbers in a shorter timespan.