Unless there is a reason why C must not be able to see the topic in question, I think I would just move the topic to your one common category. It might just be noise for people in C but creating more restrictive categories (e.g. for A+B) and educating your users on their use would quickly grow in complexity.
A, B and C has A+B, A+C and B+C. Add D and you add A+D, B+D, C+D, A+B+C and so on.
You could create tags like for-A, for-B, etc. so that when you move a topic like this to your common category, users can identify at a glance whether or not it is relevant to them.