USA (or NATO) has already provided Ukraine with a financial aid of 350 million dollars. They can't interfere militarily, it could potentially cause a wider outrage and even risking WW3. In general, I have mixed feelings about this situation (of not interfering), on the one hand, Ukraine has been left alone to deal with one of the most powerful nations, on the other hand, any interference could lead to a third World War and who knows what else, it's certainly a double edged sword.
I don't think USA will directly join the war. This is simply because Ukraine is not yet a member of NATO. Although, they can probably donate money, food and weapon to Ukraine but won't get directly into the war because as you stated it might probably lead to WW3. Is so unfortunately that Ukraine will have to face Russia alone, the war has already done them more harm. But, I think Putin don't just want Ukraine to join the NATO, he sees the organization as a serious threat.