RewriteCond %{HTTP_REFERER} !^
http://(www\.)?mysite 1(/)?.*$ [NC]
Just copy that line over and put in the domain of the other sites you want to allow.
Like So...
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^
http://(www\.)?mysite 1(/)?.*$ [NC]
RewriteCond %{HTTP_REFERER} !^
http://(www\.)?mysite 2(/)?.*$ [NC]
RewriteCond %{HTTP_REFERER} !^
http://(www\.)?mysite 3(/)?.*$ [NC]
RewriteRule .*\.(gif|jpg|jpeg|bmp|)$ 
http://www.google.com [R,NC]