I'm not a tax pro and still getting others to take a look, but technically it's 3 years from the date you filed the return, and not the due date.
Then, they have a lookback period of 3 years (plus extension if you automatically qualified as expat or applied for the october one) BUT they'll only refund you what you paid during the lookback period.
https://www.irs.gov/filing/time-you-can-claim-a-credit-or-refund
If you filed a return 10 years after the April/Oct due date and made payments 2 years before filing, you'd still be able to get a refund from that payment.
