- Do a backup (to google drive if possible) and select all of the options.
- Verify the backup
- Download the backup to your PC
- Create the required firewall rules for your Google Cloud instance
- Create your new debian instance (which you can name yourself) on your own google cloud account (free trial will give you approx 1 year for a normal server). I recommend a custom build with 1 vCPU, 2 GB of RAM or more, and 40 GB persistent SSD. It's pretty cheap (about $30ish a month) and cheaper yet if you commit to long term use.
- Go through the process of installing 3CX manually following these instructions:
https://www.3cx.com/docs/manual/installing-debian-linux-pbx/
- When you're installing, select restore from backup. Select the backup you downloaded.
- Wait for the process to finish.
- Clean up the loose ends (updates, etc)
- Ask 3CX to delete your instance from their server (if you're a nice person

)
- Enjoy your own hosted server.
NOTE:
You /can/ use 3CX's PBXExpress link to automate the installation to your own google cloud account. It's pretty easy to do BUT it will automatically create an obnoxiously long server name. If you're ok with that, the PBXExpress link will also create the firewall rules for you.
Good luck!