Do I have to pick up my baggage or go through immigration formalities before I transfer to San Francisco?

No matter which airport you are at, you have to go through the immigration office first, then take your luggage and go through the customs. So get off the plane and line up at the immigration port as soon as possible. It usually takes about an hour. When queuing for immigration, luggage will also appear on the turntable one after another. Baggage is usually slow, taking more than an hour. So it's just right for you to wait for your luggage outside after you pass the immigration office, which will take about 20-30 minutes. This time is not necessarily, it depends on whether many flights arrive at the same time when you arrive. If many international flights arrive together, the immigration office and the baggage carousel will have to wait for a long time. But I think you have more than three hours. I don't remember whether United Airlines should take down its luggage after the stop. If necessary, it is to move the luggage from the international turntable to the domestic transfer turntable. The airport will have instructions. You can also ask someone at the airport.