If I wanted to work here part time, would it be possible to work just three 8 hour shifts a week? Are they flexible?

Yes but get hired first and let HR be aware of your peculiar problem why you will prefer working on the ' flexible shift'. Its all about your peculiar problem. You can also ask if you could be allowed to work part time for an 8 hour day if that is not available. If it's available you've won the day. Just ask the HR team who are well trained to listen to you and to answer all your questions. Please lam not talking for Amazon, but having been working there for almost 3 years, l see and hear and so knows most of what happens there. Good luck .

Yes you can do that ! They very flexible. But you need to get hired first.