The US cannot win a three front war, that is why when things heat up in the Mid East and in Taiwan you can be sure they'll leave Ukraine to NATO.
If you mean at present we do not have the manpower to fight that many fronts I agree. However, we fought on more fronts than that in WW2, Western European front, Mediterranean front, Pacific Island front, Northern Atlantic front, and the IO or Indian Ocean front.
We of course had the personnel to do so but expect if we were to find ourselves fighting on multiple fronts today we would raise the personnel to do so.
I do not find Taiwan would be that hard since we are using a first island and second island line strategy. Which of course is more to hold China in and bottle them up. I find the Corps is ahead of the curve by currently retraining like we did in WW2 with a Island hoping strategy. As well as we are calling in old treaties and wanting to build additional airfields on different islands in the Pac.
This way we can project additional air power beyond just our carriers. I don't believe we would fight Russia through Ukraine, I am sure we would put boots on the ground to keep up with any plans NATO would have. However, the best place for us to open a front would be in the North Lant. Attack Murmansk with Naval and airpower while you do a land attack in the Kola Peninsula.
I do agree at present we lack the manpower to open multiple fronts to fight a conventional war.