Tony and Dean together read 23 + 12 = <<23+12=35>>35 books
3 of these books were duplicates so Tony and Dean together read 35 - 3 = <<35-3=32>>32 unique books
All three read 32 + 17 = <<32+17=49>>49 books
Because the books are being counted three times, the book is counted 3 - 1 = <<3-1=2>>2 extra times.
All together they have read 49 - 2 = <<49-2=47>>47 different books
#### 47