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Google allows refunds within the first 15 minutes of purchase. Push the refund button from the Google Play store. After the 15 minute window refunds can be requested from Google via a web form here: https://support.google.com/googleplay/contact/apps_refunds_payments_issues/
Amazon does not have a refund policy. You would need to contact their support team to request a refund.
There are multiple ways to request a refund with Apple, all of which are outlined here: http://www.imore.com/how-to-get-refund-itunes-app-store