SELECT COUNT(DISTINCT a.col1)
  FROM tbl1 a,tbl2 b,tbl3 c
 WHERE a.col2 = b.col1
   AND b.col2 = c.col2
   AND fn_mth(a.col3,a.col4,5) = 12
   AND a.col5 = :input_value 
   AND c.col5 = 0;

SELECT Count(DISTINCT a.col1)
FROM   tbl1 a
WHERE  a.col5 = :input_value
 AND   a.col2 IN ( SELECT b.col1 
                   from tbl2 b
                   WHERE b.col2 IN (SELECT c.col2 
                                    from tbl3 c
                                    WHERE c.col5 = 0
                                   )
                 )
 AND   Fn_mth(a.col3,a.col4,5) = 12