Age of Empires III: Definitive Edition DLCs are usually enabled automatically after purchase. However, if that doesn't happen, they can be enabled manually. The steps vary slightly depending on the store the game was purchased from. 

After August 13th, all players who have purchased Age III: DE will be automatically gifted three DLC if they are not already in their collection. 

Details about the DLC included and other updates can be found in the blog:

Celebrating Age in 2026 – Where We’re At, and Where We’re Going - Age of Empires - World's Edge Studio

XBOX PC APP

Follow these steps to view the list of DLC and enable through the XBOX PC app. 

1. Right click Age of Empires III: Definitive Edition in the XBOX PC app. 
2. Choose Manage and then select ADD-ONS.
3. Check the boxes next to each Add-on to install the DLC. 

More detailed steps on installing and uninstalling games can be found on the XBOX Support site. 

All about the Xbox app for Windows | Xbox Support

STEAM

Follow these steps to view the list of DLC and enable through Steam. 

1. Go to your Library in the Steam client. 
2. Right click Age of Empires III: Definitive Edition and select Properties. 
3. Choose DLC from the list. 
4. Check the boxes next to each DLC to install. 

These steps can also be found on Steam's website. 
Steam Support :: Find Downloadable Content (DLC) for My Games