How can I get a copy of my receipt for a macbook air from 3 years ago?

You can contact Apple, but I do believe they only make the last 18 months available. If from a different vendor, contact the vendor. Other than that, check your email backups if there is still a copy. And, for future reference: create a folder in your User folder with "Apple receipts" and, every time you buy something, make sure to put a copy of the receipt in that folder.