PhD applications: ethics of not informing other universities after accepting an offer
I think you should withdraw your applications from B and C.

By keeping your application active, not only are you wasting the committee's time, you are potentially hurting other students.  Most graduate programs can only accept a certain number of students per year, but there may be many more applicants who would still be well qualified and able to succeed.  If the committee accepts your application, that means they are denying admission to someone else who might be equally deserving.  If you withdraw now, you open that space for someone else.