I can confirm that 3 in Austria gives you public IPs (not NATed). However, such public IP is protected by the provider (3) by default so that no connections from outside may reach your device. However on the 3 website you may uncheck this. It may take some days for 3 to make this change to take effect but it will allow you to have a mobile server. I tested this using some IP Webcam. It works fine.
Let me also mention that there is a "Port Forwarder" app in the market that allows you to forward a port to a device in your local subnet that you created using any tethering method.