Dear Friend,
MVC and 3-tier architecture are different things all together. If they were the same, then anybody with a 3 tier architecture would by definition have implemented MVC. Since that would be a false statement, we can safely say that they are not the same thing.
MVC has specific architectural patterns as to how the tiers interact that differ from a straight 3 tier architecture.
http://en.wikipedia.org/wiki/Three-tier_(computing)[
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I hope this will clear your confusion.