Three Tier comprised of Presentation, Business, Data layer.
Presentation Layer: This layer top layer in an application. It defines User Interface(UI) to user for performing its activity.
Business Layer: It is middle layer between Presentation and Data layer. It communicates both layers. It takes input from Presentation layer and send it to Data layer. Then returns result to Presentation layer.
Data Layer: This layer is nothing but performs only database operatins. It holds application data.
Please go through below links that might help yo understand batter:
Create-and-implement-3-tier-architecture-in-Asp-Net[
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