如何在wordpress中使用rss检索不同类别的内容？

There is a wordpress web site which has categories news (514), announcements (1370), culture (770), sports(771), education(772). Numbers are the category ids.

This web site gives RSS feeds and I can retrieve news contents using http://www.example.com/?feed=rss2&cat=514 and so on.

I want to grab all three culture, sports and education contents at once.

I have tried some urls like

http://www.example.com/?feed=rss2&cat=770,771,772

and

http://www.example.com/?feed=rss2&cat={770,771,772}

but did got only the first id's contents.

Is there another solution for this problem?

PS: I am not managing that web site. So I cannot make settings on it.