The main game installs ok, but when I try to install any of the 2 dlc''s the gog installer stops with the message that it cannot detect the AoW3 GoG install. Game is playable but it lacks the 2 extra scenarios.
Moreover, the key provided by GoG only registered as AoW 3 within Triumph launcher, no mention of being the deluxe edition (i saw that the steam edition lists the 2 dlc's as separate entries in the Triumph launcher).

Any workarounds to install the 2 dlc's and properly register them with Triumph?

Funny, they should have just packaged the DLCs as a seperate product.

The DLC install issue was resolved by re-downloading the main game installer.
Apparently the 1st download was corrupt. The 2 DLC's appear OK in game now (but still not showing up in the triumph launcher).

Hmm maybe I'll have to re-download myself as I'm not seeing the DLC from the pre-order deluxe version in my game either. I was just playing thru the first recommended campaign thinking maybe it unlocks after you do one campaign first. Time to re-download, thanks for the tip.
I also noticed triumph is not acknowledging mine is the deluxe version either which I thought odd.

The 2 DLC show up in the scenarios maps, as two additional scenarios. For the pre-order deluxe edition, you should have a total of 10 scenarios listed (8 from regular game + the 2 dlc).