what is the difference between 3 tier architecture and a mvc?

MVC is a pattern used to make UI code easier to maintain and test. When the MVC pattern is used a larger portion of the UI code can be unit tested.

Here is a good article which describes the MVC pattern in more detail: http://martinfowler.com/eaaDev/uiArchs.html

3 tier architecture is a pattern used for a completely different reason. It separates the entire application into meaningful "groups": UI, Business Logic, Data Storage.

So 3 tier application refers to all code in the application. The MVC pattern is a pattern used in the UI tier.

Here is a good article on the 3 tier architecture: http://dotnetslackers.com/articles/net/IntroductionTo3TierArchitecture.aspx

For further information you can search the internet and find a gazzilion articles on both subjects.

Comparison with the MVC architecture

At first glance, 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 [model-view-controller] 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.

Source: http://en.wikipedia.org/wiki/Multitier_architecture#Three-tier_architecture

In MVC : 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

In Three Tier : 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