This is the basic implementation of 3 tier architecture in visual c# using windows form application.

Procedure:

Create a new windows for project and name it APP_LAYER
Add a textbox and a button to the form1.cs
Add a class library to the Project and name it B_LAYER
Add another class library and name it D_LAYER
Right click references in APP_LAYER, provide it reference of B_LAYER
Right click references in B_LAYER, provide it the reference of D_LAYER
This will link class libraries with eachother.

Create relevant functions in the class libraries as in the project and execute the code.