In real time scenario adding two folders in App_Code is not considered as three tier architecture. You have to create two different class library project and then add reference them from your application For example: Create a class library project like YourProjectBusinessLayer and another class library project as YourProjectDataAccessLayer. Add a reference to YourProjectBusinessLayer for YourProjectDataAccessLayer and a reference to your web application project for YourProjectBusinessLayer.
This will make the hierarchy something like:
YourProject->YourProjectBusinessLayer->YourProjectDataAccessLayer
Hope it helps.
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and
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are good examples to start.