What page is "?page_id=3&category=10"?
What's the error, exactly?
If it's a category, that's a wrong way of accessing category pages.
If you changed the URLs to pretty permalinks, you must be sure to use the URL functions on all links in your theme. Like get_permalink() or get_category_link().
Also, you must have mod_rewrite enabled in your server (check phpinfo() for that), and the .htaccess file in the root of your WordPress installation must have the required rules (see the permalinks option page, it gives you the .htaccess content if there's no server writing permission for the file).