what is 3 tier in asp.net

There is not to worry about it. It is a simple task.

Make a class for DBconnection.Make connection string in a mehtod returing the value .
Make a class for DBAccess-For qyeries (insert,delete,update,procedures).
Make a class for BAccess (Business Login)-call DBAcess and passing through parameter.
Last=From code behind pass values in BAAcess required function.

Simply just divide your application in three tiers as

1. Presentation Layer
Which will contain all the thing which makes your project to be presented from front side all types of css,Skins,Controls you have used etc.

2.Business Layer
Which ensures that the data inserted in your presentation side is Valid upto.Use all kind of business validation here.

3.Data Layer
All the data related task weather connecting to database and retrieving any data values.

You could have THIS[^] Link to be useful for further information.

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