3-Tier Architecture means a
User Interface Layer - For interaction(GUI Forms)
Business Interface Layer - to handle the business logic (Calculation part ) -BI
Data Interface Layer -- To handle the Data i.e., From Database Connections, Calling StoredProcedures, - DI
Info Layer -- To handle the Get,Set Properties mostly common
Mostly BI & DI are the class libraries build into the projects or components if used in multiple projects
The main purporse is to maintain the Code in an understable format for developers worldwide, easy maintainance.