Final answer:
The application tier in three-tier architecture handles the execution of business logic and acts as a mediator between the user interface and data storage.
Step-by-step explanation:
In the three-tier architecture, the function of the application tier is to execute the business logic of the organization operating the database. This layer acts as a mediator between the user interface and the data storage, handling processing, logic, and rules of the application. It is responsible for the coordination of the application, processes commands, makes logical decisions and evaluations, and performs calculations. It also moves and processes data between the two surrounding layers.