I've searched and searched, and I know this has been answered, but apparently I am not using the right search terms. There must be an easy way for www.domain.com, www.domain.net and www.domain.org to all be served by the same ispconfig website, but I can't find it.
Create a website for domain.com, set auto subdomain to www. then add a aliasdomain for domain.net an domain.org to that website, set auto subdomain in both alias domains to www as well.