left joining and combining the results where they are not null
Without an actual query attempt from you, some thing are difficult to understand.

From what you've said, you're looking to group by date only, not on any information from c. That being the case, it's not clear that c needs to be in the query at all.

The following query should give you the totals you want, for all dates found in either c_r or c_s:

SELECT c_date
      ,COALESCE(SUM(r_amount),0) as r_amount
      ,COALESCE(SUM(s_amount),0) as s_amount
  FROM (
        SELECT c_date, 0 as r_amount, something_numeric as s_amount
          FROM c_s
        UNION ALL
        SELECT c_date, something_numeric as r_amount, 0 as s_amount
          FROM c_r
       ) sq
 GROUP BY c_date
;


If you need to limit rows by some value in c, INNER JOIN each of the tables in the subquery to c separately, with identical WHERE clauses. You could alternately include the values from c in the subquery's SELECT list, and filter in the main query, but that's probably less efficient.