Two days later, mainland China attacks air bases along the Northwest coast of Taiwan.
Simultaneously, Iran launches air strikes at civilian targets in Israel, knocking out major portions of Jerusalem.
Two days after the Chinese and Iranian strikes, North Korea opens up a massive artillery barrage on Seul.
How does the United States respond to these four simultaneous threats?
None of your hypotheticals are threats to the United States.
1) Eastern Ukraine is part of Russia. Western Ukraine is part of Poland, or Austria. Interference from Western powers is the cause of a Ukrainian state, and Stalin’s annexation of Lwow/Lemberg and hinterlands is the cause of a irredentist pro-NATO/EU faction.
2) Taiwan has been acknowledged as part of China by the US since 1983, and realistically since 1973. The internal affairs of China are none of our business, unless ours are also the business of China.
3) Israel is, in the long run, strategically indefensible, If it’s not Iran, it will be somebody else. In my opinion, the Jewish residents of Israel should leave while they have the chance but that’s up to their prudential judgement, not mine. Again, no American interests are involved.
4) North Korea would face a regime crisis that they probably could not survive if they attacked South Korea. I doubt that they would have Chinese help unless the US was using Korea-based forces to interfere in Taiwan, which teh Seoul government would never allow.