To upgrade your gluu from 2.4.4 to 3.1.1, you can follow these steps in [this documentation](https://gluu.org/docs/ce/3.1.1/upgrade/#upgrade-from-2xx-to-311).
That being said, you must install 3.1.1 on other servers and use your former gluu server as an NGINX/Load balancing proxy, per [these instructions](https://gluu.org/docs/ce/3.1.1/installation-guide/cluster/).
The logic behind that is having NGINX or some load-balancer/proxy to forward requests from 1 domain will allow you to access oxTrust/oxAuth on the other domains, due to certificate routing. It also allows failover if one of your nodes goes down, your other servers will be actively handling requests and authentications.
So the process should go something like this:
Follow the upgrade instructions until you have to install 3.1.1. Install 3.1.1 on a new server, then copy the backup data from you 2.4.4 server to the 3.1.1 server. Finish the upgrade instructions on the 3.1.1 server.
After you've imported all your old data, follow the instructions from the manual clustering documentation for the rest of the nodes.