Save Game File Location
Age of Empires III: Definitive Edition save games can be found here:
Save Data Location – *We do not recommend removing or altering save data in this folder*
%USERPROFILE%\Games\Age of Empires 3 DE\<user-id>\saves\
Note: Sometimes you need to show hidden folders in Windows. It is important to enable Show hidden files, folders. To do that;
-Open File Explorer from the taskbar.
-Select View > Options > Change folder and search options.
-Select the View tab and, in Advanced settings, select Show hidden files, folders, and drives and OK.