I guess you do like this:
Fragment fragmentA = (getFragmentManager().findFragmentById(R.id.fragmentC));
Fragment fragmentB = (getFragmentManager().findFragmentById(R.id.fragmentB));
FragmentTransaction ft = getActivity().getSupportFragmentManager().beginTransaction();
ft.remove(fragmentA);
ft.remove(fragmentB);
ft.commit();
And then just add new fragment.
getSupportFragmentManager().beginTransaction().add(R.id.container, fragmentC).commit();