Check whether there is a "DLC" option in the main menu right after starting the game and go through it, if possible.
If you use Galaxy, you can also go to Risen 3: Titan Lords -> Settings -> DLCs. There should be a "Risen 3 - Titan Lords Complete" checkbox or something like that.
Apparently I'm out of luck - no Risen games come up in the "store" - I guess GoG no longer sells them. I do have 1.2 EE - is that the same thing? If so, how can I activate the additional content?
EDIT: well, it's not the same thing as "EE" means enhanced edition (which I have no clue as to how it's different from the unenhanced edition). I may be able to order it anyway, though - while nothing came up for Risen, Risen 3, or Pirahna Bytes via search on the "store" page, I was able to get to it and it appears it is orderable from the "store" link on the Galaxy page for the game in my library.