I really do not want to see how all of this plays out, but I believe China has also quite some manpower and their military capabilities are not exactly known. The outcome will likely depend, how the rest of the world reacts. And if there are nukes.

*could. It seems to have lost the appetite recently, and bankrupted itself with the twenty year war on terror against itself, which it lost.

>US can fight three peer

That's never been doctorine for the simple reason US hasn't had (near) peer rivals until PRC in last 30 years. Peak hyperpower US 90s doctorine was calibrated for 2 "major" wars, when major is with adversaries who were frankly all medium powers (IRAQ tier). Then in 00s-10s it shifted to 1 "major" war, 1 holding war, i.e. actively fight 1 major war, fix another major war in place so resources can shift to second war after active war. Now half the think tank writing is questioning if US can win in PRC backyard, where PRC is characterized either as near peer, pacing power, or peer. US doctorine right now is maybe can deter PRC until post 2030s "decade of concern" but right now US IndoPac posture in PRC backyard a tossup.

I think they now have 3 carrier groups out in the world at the same time. each of these is basically enough to best a foreign military excluding nukes.

Gulf war against 90s Iraq with 20m people and ~800 1960s missiles (scud Bs) required 5 carrier groups + regional air basing for air campaign, completely compromised Anti Air (by french who designed Iraq IADS and gave schematics to US). Relative to modern Iran with 80m people and 4x more missiles with 2000s rocketry tech, much more favourable geography (facing water), 1990s Iraq/Gulf War would not even measure up as a medium power / major war. One (1) carrier with 150/270 (short term surge) sorties per day is not remotely enough against a medium power. Can US surge 6-7 carriers + forward deploy air force with advanced notice to tackle Iran, yeah, but it wouldn't leave much left for anything else. CVN69 isn't exactly defeating the houthis right now. That's the numbers behind why US doctorine downgraded to 1 major war + 1 holding war (with medium/small adverary), while near/peer war with PRC is currently a toss up.