Check the view you are using for your A datasource. The view specified needs to contain B and C as well as all of the attributes from B and C that you want to display.
Let me clarify, say I have another entity called ‘Master’ with a one-to-many link to A. So the reference is only against the Entity A, not to B and C. So, how do I go about exposing the attributes of B and C to Master? I have a view in Master but I do not / cannot see the attributes of B, C.
If you have one-to-many reference from Master to A and many-to-one references from A to B and C, you should be able to select in a Master view first A attribute and then its B and C attributes. Or first define a view for A containing B and C (say a-view), and then select this view for A attribute in the Master view.