Hi, sorry if this is a silly question. I wound up with two art books and thinking I had received it by mistake checked my pledge - I did order two, so that's on me, but I'm happy with it anyway as a gift for a friend! Beyond that though, I noticed the pledge had a line on Backerkit that reads: '"Threes" single' (1), book (1), art book (2), audiobook (2). What does the "THREES" refer to? Thanks!