Exclude composite key from select
I think you need this:

Select * from b.table as b 
where  not exists                
     (SELECT 1
      from A.table as a 
      where a.C1=b.C1 and a.C2=b.C2 and a.C3=b.C3 ) ;



You shouldn't add b.table in the subquery's from clause, because you already have it under the b alias in the main query.
You don't need to specify columns in the subquery's select clause, because that doesn't have any effect. "Is there a row satisfying the where clause?" – that's the question that the exists predicate is meant to answer.
You need AND between the conditions in WHERE, not commas.