In Two Tier application if client hits the server then in 
Server the request goes, verified by the Databse or 
buisness logic as application layer and Database are 
connected in parallel and server replies to the client.
In Three Tier architecture when client hits the server then 
in server there is some temporary database connected in 
parallel with Application layer and after that the main 
Database server is connected with that server in series, 
after verification from the first database server
(Temporary) the data finally goes to second Database server
(Final repository).
This is how the architecture can be increased to n-tier.