Go to Single Player > Skirmish. In the map selection dropdown, change the category from "Standard" to "Custom". Your new maps will appear in the list.
I can provide direct links to the best map packs based on your preferences.
Navigate to your Windows user directory: C:\Users\[YourUsername]\Documents\My Games\Age of Empires 3\Scenario . Drop the .age3scn file into this folder. How to Load Custom Maps In-Game