Actually, you need something like this. Allow/Deny won't work for what you want:
AuthUserFile /dev/null
AuthGroupFile /dev/null
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^
http://www.site-a.com/.* [NC]
RewriteCond %{HTTP_REFERER} !^
http://www.site-b.com/.* [NC]
RewriteCond %{HTTP_REFERER} !^
http://www.site-c.com/.* [NC]
RewriteRule /*
http://www.yoursite.com/error.html [R,L]