MVC is all about seperation of concerns - but deals with specifics in that the View is loosely-coupled to the model and controller; with 3-tier this is not explicitly required.
Yes - you can (and should) have all three tiers loosely-coupled but that's not explicitly required by 3-Tier; with MVC it is.
As an aside: MVC is a specific pattern that looks at a specific issue, I would be careful when referring to it as an "architecture".
An advantage of Microsofts ASP.NET MVC implementation is that is offers various integration points and flexibility which aren't so easily accessible with "standard" ASP.NET - which I guess you could say was/is traditionally used when build web-based 3-trie systesm in the MS space.