Pierre,

Since you already have the primary keys setup in the tables the only additional key you need is between tableA and tableC. Ideally you need to put the PK of tableA in tableC. Once done, use the database designer. Open the designer exclusively and using the mouse make your visual relations (not sure if you know how to do this - if not, just ask). Double click on any of the relations and click on the RI button. You'll see your options for adding, updating and deleting at this point.