3 Tier architecture is structural pattern of an application , where 3 layer works separately like Business layer , Presentation layer and Data layer

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.