I’ve paid using PayPal pay in 3 how do returns work?

If you haven't paid all of your instalments yet:

If you haven't paid all three of your instalments yet, the monetary amount of the product you are returning with be deducted from the remaining instalments you owe, meaning you will pay less on these instalments.

If you have paid all of your instalments:

If you’ve already paid instalments and your refund amount is more than the balance remaining, your PayPal account will be refunded.