3 - Tier Architecture is like following :
1. Presentation Layer
2. Business Logic Layer /Data Manager Layer
3. Data Access Layer
The communication between all these layers need to be done using Business Entities.
1. Presentation Layer is the one where the UI comes into picture. Presentation layer contains pages like .aspx or windows form where data is presented to the user or input is taken from the user.
2. Business Logic Layer is the one where all business logic are implemented. It contains business logic, validations or calculations related with the data, if needed.
3. Data Access Layer is the one which communicates directly to the database. DAL contains methods that helps business layer to connect the data and
perform required action, might be returning data or manipulating data
Data from one layer to other needs to be transformed using Entities.