How to make OS X Server 3 accessible from the internet?
Essentially you have to make your server accessible behind a router  with a frequently changing IP-adress with a fixed fqdn for the router and forward all http/https requests from the WAN to the OS X VM in the LAN.


Set up dynamic DNS:


Get an arbitrary name from a dynamic DNS service provider (like Dyn (not free) or NO-IP (free))  
or depending on your hosting service and the access level you have, set up your own DDNS service

Set up a DDNS client on your OS X Server
Set up your OS X Server VM with a fixed IP
Set up and check port-forwarding (default is 80/443) on your Airport Express (some firmware versions seem to be buggy) to the OS X Server VM.
Access your home web server  from the internet with the chosen fqdn (e.g. (www.)myserver.no-ip.com).
Consider setting up some countermeasures against evil attackers.