Query.sql

SELECT groups.id, groups.created_at AS group_created, MAX(gu.created_at) AS newest_user, MAX(gp.created_at) AS newest_photo, GREATEST(groups.created_at, IFNULL(MAX(gu.created_at), 0), IFNULL(MAX(gp.created_at), 0)) AS latest_activity FROM groups 
	LEFT OUTER JOIN groups_users gu ON groups.id = gu.group_id
	LEFT OUTER JOIN group_photos gp ON groups.id = gp.group_id
GROUP BY groups.id

Results.txt

id	group_created		newest_user		newest_photo		latest_activity
1	2009-12-01 18:39:25	2010-07-20 21:23:50	NULL			2010-07-20 21:23:50
2	2010-06-14 15:28:21	2010-06-14 15:34:01	NULL			2010-06-14 15:34:01
8	2010-07-15 16:14:47	2011-06-23 18:50:52	2012-09-05 17:03:09	2012-09-05 17:03:09
9	2012-09-05 18:48:07	NULL			NULL			2012-09-05 18:48:07