[AS3] understanding

I have been reading up on AS 3 and looking at some of the tutorials and can’t seem to understand something. When I build sites in AS 2 I have a main class and most of my classes filter into the Main class. I rarely link a class to a MC in my library and instead just attach the MC in my class and reference the attached MC.

Now should I approach AS 3 the same way, or should the code be linked to a MC?