Simply add three items from the collection to your basket. No need for a code... the discount will automatically be applied at checkout. The discount will be placed on the lower value item, and will be added multiple times for multiple purchases.
Don't forget that We Buy Back all items, even those purchased on sale! Buy Backs are even more significant when you've bought the pieces on sale as we always use the original value to calculate the pay out amount and not the sale price you paid!