The 3-Tier architecture has the following three tiers:

Presentation Tier: This is the top most level of the application. The presentation tier displays information related to such services as browsing merchandise, purchasing, and shopping cart contents. It communicates with other tiers by outputting results to the browser/client tier and all other tiers in the network.

Application Tier (Business Logic/Logic Tier): The logic tier is pulled out from the presentation tier and, as its own layer, it controls an application’s functionality by performing detailed processing

Data Tier: This tier consists of Database Servers. Here information is stored and retrieved. This tier keeps data neutral and independent from application servers or business logic. Giving data its own tier also improves scalability and performance.

Source: Wikipedia