Object oriented question

I have a question regarding to object oriented Actionscripting.

I have a class A, which is the document class. Then I have a class B and class C. I create an instance of class B in the document class like this
[AS]
var _b:B = new B();
addChild(_b);
[/AS]
in class B I create an instance of class C. In class C I want call a method that is defined in class B. How do I do this? As I understand it, I can’t create another instance of class B inside of class C, because I have already done that in class A. So in class C this
[AS]
var _b:B = new B();
_b.someMethod();
[/AS]
isn’t acceptable? How do call class B’s method from a class that is created in class B?

I hope this makes sense to some of you :sen: