Sorry to hear you missed out on the 3 for 2 offer. We cannot add on any items in retrospect that customers haven't selected before checking out. You'd need to select 3 items which are in the 3 for 2 offer, before placing your order, to redeem this offer.
Please remember to check your basket at checkout to ensure you've qualified for the 3 for 2 and do not miss out next time.