Forum Moderators: open
This isn't working because Google defaults to AND. What you're saying to Google is this: "I'm looking for keyword foo and the result pages have to come from foo1.org AND foo2.org AND foo3.org." Obviously a page resides on only one domain so you are not getting any results.
Instead, you need to use OR. I find ¦ quicker to use than typing out OR. So your query should look like this:
myword (site:foo1.org ¦ site:foo2.org ¦ site:foo3.org)
Hope this helps!