Can I return used AirPods 3 for a refund?

If you have purchased your AirPods directly from Apple, in most countries/regions Apple offer a generous returns policy; you have 14-days within which you can unconditionally return an unwanted item (including AirPods that have been used) for exchange or refund.

The item being returned must be undamaged and include all components and packaging - and must be accompanied by the original purchase receipt that shows the serial number of the returned item. All Apple-issued sales invoices include this information. Beyond 14-days, unless defective, the opportunity to return an unwanted item will be lost.

If instead you purchased from a third-party retailer, you will need to explore the retailer's own returns policy.

Extremely unlikely Apple would allow you return an item that has already been in your ears.

You can ask, but don't be surprised if they turn you down.