What should I do if a filling falls out? I had what I thought was a piece of tooth crack off, but now that I look at it I think it's a porcelain filling. There's no pain, and I have an appointment with my dentist in three weeks. Would it be okay to wait u

Call dentist. Let your dentist decide if you should be seen sooner. My advice would be to have problem addressed asap. Unfortunately many dental problems don't hurt until they go from minor to major.
Don't wait . Call your dentist and get this tooth checked sooner than your appointment. Just because there is no pain now this does not mean you are safe. It can keep breaking if you use this to bite or chew food with, so be careful.
It . It is hard to figure out from your description if it is a crown that broke or the tooth itself... Call your dentist and have him help you make the right decision. Please call your dentist.
No . No reason to wait as absence of pain is not always an indication of absence of a problem. Call!
If . If the crack is significant and the sharp edges is irritating your tongue or cheek, it is wise not to wait till your next appointment in three weeks. It can't only cause sore that can take 7-14 days to heal and cause breath problem but can affect your occlusion (bite) as well.