What is schema?
   
           The logical structure of the database is called schema.
 
            Schema may be categorised by------
      1. Physical schema:-
                                             Database design at physical level.
      2. Logical schema:-
                                           Database design that logical level.

 What is instance?

            The actual content of the database at a particular point in time called instance. It is a analogous to the value of variable.

Three   schema  architecture or architecture of DBMS?