Before moving to understand what is difference between 3 Tier Architecture and 3 Layer architecture lets have an overview about what is Tier and What is Layer ???
The layers reside on the same machine where the tiers can reside on different machines.
Layer communicates with each other either by Value or by Reference. However the Tiers could be on different machines, so they communicate by Value only – as serialized objects.
Layer is logical separation of code and Tier is physical separation of code, In other words we can say Tiers are the physical deployment of layers. Or a Tier becomes a Layer if it could be physically separated from the Layers it consumes.
N-Tier Architecture is a physical structuring, while a N-Layer Architecture is a logical structuring.
Now coming to 3 Layer Architecture and 3 Tier Architecture after getting an idea about Layer and Tier.
In 3 layer architecture, the Database Access Layer (DAL), Business Logic Layer (BLL) and User Interface Layer (UIL) resides as 3 different project and the output of these 3 projects (.dll file) must be together in the same server or on same machinein order for the system to run.However in 3 tier architecture, the Database Access Layer (DAL), Business Logic Layer (BLL) and User Interface Layer (UIL) reside as 3 different projects. But each of the projects can be deployed at the different server or at the different machines and distributed functionality is explored.
In 3 layer architecture DAL, BLL and UIL can work on same machine where as in a 3 Tier architecture a client in on one machine, the application Server is hosted in another machine and the database server resides in another machine i.e. Three-tieris a client–server architecture in which the User Interface, Business Logic , Data Storage and Data Access are developed and maintained as independent modules on separate platforms.
In 3 Layer we have distinct namespaces and classes for the various layes i.e. we’re only talking about logical organization of code and passing objects between layers is easy as all assemblies will run under same application in single machine.But 3 tier applications are not straight as 3 layers, because each layer will be deployed in different machines. So here, we need a mechanism to send the objects between different machines over the network hence we use technologies called Web Services, Remoting or WCF, etc. in order to implement 3 tier applications
3 Layer Architecture will improve readability and reusability, Minimizes the application changes due to impact of the changes in other layers. While 3 Tier Architecture has all advantages of 3 layer + scalability as application will be deployed in different machines so load will be shared among the tiers and scalability will increase