Sharing elements across projects?

Best practices question.  Let's say we have 3 products.  Our Base product already has use cases written for it.  Now we are expanding to add another two related products A and B, which will have some of the same functionality as the Base product but also additional use cases to describe their unique functions.

One obvious thing to do is to create new projects for A and B, copy across the use cases from Base that are the same and add the new ones.  My concern is that if something changes in the Base use cases we then have to change it in 2 other places as well.  Therefore I am wondering if there is some smarter way to do this?  Can project information be shared in any way? Maybe using separate top level packages to represent the unique and common functions (Base, BaseA, BaseB, A, B) and functional subpackages under those? 

Just wondering if anyone out there has tried anything similar.

Thanks!