In this JLPT N3 audio practice question (#3), the correct reply to the statement is 3.
If the question translates as "How about another cup of coffee?" then I'm confused why 3 is correct. First, I thought 「いただきます」 as a set phrase only applies just before a person is about to put food to mouth; second, in this setting that I assume to be a customer and server, I'm confused why the customer would be speaking keigo. Obviously there's some detail I'm missing here.
Also, why is answer 1 incorrect? If 「結構です」is an acceptable way to turn down an offer, why would a slightly less confident 「結構だと思います」 be out of place? Is 「結構です」strictly a set phrase that can't be modified?