First, try creating a report with the Joined format. A joined report will have two reports with the same "base"/common data -- i.e. the records in the Master-object (object
A in your example) but each separate report (that's joined at at the end by the common Maser-record) is about one of those Master-Detail Relationships.
You may be able to do this without using the Joined format if you are able to create a Report Type that encompasses the three objects in question.
Try to create a
Report Type, go to Setup ---> Create ----> Report Types . Have the new
Primary Object be the Master Object (object
A in your example), and try to add both objects as secondary/tertiary objects.