Unless expressly communicated, discount codes can not be used when making a 3 for 2 purchase. If you return the free item, no refund will be issued.
If you return one of the other items, you will be refunded the amount for that product, less the value of the free product at the time you purchased it. For example, if your basket contains items for £130.00,£105.00 and £64.00, you will not pay for the €64.00 product. If you then return the £105.00 product, your refund will be £105.00, less the £64.00 for the free product, so the total refund will be £39.00.