You will need to add three Insert Record Server behaviors to the page.
in the first and second Insert Record Server Behaviors, make sure not to set a redirect page.
The first one should insert to the parent table. In the Session Variable section, set the primary key column to create a session variable that can be used in the other two to relate the records.
In the Other Tow Insert Record Server Behaviors, bind the Foreign key Columns to the session variable crated by the first one.