What is the difference between MVC and three tiers architecture?

The three tiers may seem similar to the model-view-controller (MVC) concept; however, topologically they are different.
A fundamental rule in a three tier architecture is the client tier never communicates directly with the data tier; in a three-tier model all communication must pass through the middle tier.
Conceptually the three-tier architecture is linear.
However, the MVC architecture is triangular: the view sends updates to the controller, the controller updates the model, and the view gets updated directly from the model.

3 Tier is an architectural style while MVC is the design pattern.

In larger applications MVC is the presentation tier only of an N-tier architecture.
The models views and controllers are only concerned with the presentation, and make use of a middle tier to populate the models with data from the data tier.
MVC can also be used as the entire3-tier architecture where Views are your presentation, Controllers are your business logic and Models are your data layer (usually generated by a DAL such as Entity Framework).
Ideally though you want your controllers to be skinny and dumb, passing off logic to a 'business component', which would essentially become your middle tier.3-tier is a Architecture Style and MVC is a Design Pattern.
so is Different in that.
but we could using mvc pattern in3-tier architecture style.
so: Presentation Tier: "Controllers and Views" from MVC Pattern.
Business Tier : "Model(Data)" from MVC Pattern.
Data Access Tier : Original Data Access Tier.

chk out the following link http://en.wikipedia.org/wiki/3-tier_architecture#Comparison_with_the_MVC_architecture

the mvc used to make code easier to maintain and test but3 tier architecture used to separates the entire application to business Logic and data Storage (database) in another meaning the3 tier application refers to all code in the application but the mvc is a pattern used in the UI tier.
good luck