When asked to verify the product info, Amazon.com customer service replied with the following: "I understand that it is listed as 2 disc, 6 panel digipak in other websites and it is listed as 3 discs on our website. The only information we can give you about a particular CD is what is displayed on the detail page for that particular music title."
No further clarification was given. It is possible that they are counting the sticker pack as a disc. At some point, Amazon US was updated to show 2 discs.
The images posted on nin.com on July 16, 2008 show the label on the album which reads "limted edition CD + DVD". It is difficult to make out the smaller print, but it doesn't appear to give any further clues to a 3rd disc.