Three-tier (layer) is a client-server architecture in which the user interface, business process (business rules) and data storage and data access are developed and maintained as independent modules or most often on separate platforms. Basically, there are 3 layers, tier 1 (presentation tier, GUI tier), tier 2 (business objects, business logic tier) and tier 3 (data access tier). These tiers can be developed and tested separately.

3 - Tier Architecture is like following :

1. Presentation Layer

2.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

2. Data Manager Layer is the one where all the maipulative code is written. Basically in this layer all the functional code needs to mentioned.

3. Data Access Layer is the one which communicates directly to the database.

Data from one layer to other needs to be tranformed using Entities.