A three-tier architecture introduces a server (or 
an "agent") between the client and the server.  The role of 
the agent is manyfold.  It can provide translation services 
(as in adapting a legacy application on a mainframe to a 
client/server environment), metering services (as in
acting as a transaction monitor to limit the number of 
simultaneous requests to a given server), or intellegent 
agent services (as in mapping a request to a number of 
different servers, collating the results, and returning a 
single response to the client.

Three-tier architecture also known as MVC architecture in 
which software design is split into three groups as Model, 
View and Controller part of UI goes in model section while 
Business logic goes in controller section and data access 
layer goes in view layer. 
Now consider the view or data access layer(DAL) in which 
operations such as data access from database insertion such 
type of operations are kept in this section. In Controller 
known as business logic layer in which all logic related to 
UI and DAL is implemented here and accessed from UI using 
instance of objects. Finally in UI only UI related code is 
written here.