MySql: #1221 - Incorrect usage of UNION and ORDER BY

Question

I have the following query which is intended to retrieve the newest 3 comments for each post in an array of posts.

SELECT * FROM comment AS tbl WHERE sscm_id IN 
(
  SELECT sscm_id FROM comment AS t1 WHERE t1.sscm_oid = 4 
    ORDER BY t1.sscm_id DESC LIMIT 3
  UNION  ALL
  SELECT sscm_id FROM comment AS t2 WHERE t2.sscm_oid = 3 
    ORDER BY t2.sscm_id DESC LIMIT 3
)

which gives me the following error:

1221 - Incorrect usage of UNION and ORDER BY

Some searching & googling seemed to suggest that placing () around each subquery might solve this problem but changing the query to

SELECT * FROM comment AS tbl WHERE sscm_id IN
(
  (SELECT sscm_id FROM comment AS t1 WHERE t1.sscm_oid = 4 
     ORDER BY t1.sscm_id DESC LIMIT 3)
  UNION  ALL
  (SELECT sscm_id FROM comment AS t2 WHERE t2.sscm_oid = 3 
     ORDER BY t2.sscm_id DESC LIMIT 3)
)

but this gives me the following error:

1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UNION ALL (SELECT sscm_id FROM bpsocial_comment AS t2 WHERE t2.sscm_oid = 3 OR' at line 4

So: how can I get the UNION in combination with the ORDER BY in the subqueries to work?

Thanks R