Ubuntu Touch currently can't be installed through the installer on the Fairphone 3.
The device page for the Fairphone 3 (https://devices.ubuntu-touch.io/device/fp3/) has manual installation instructions here: https://forums.ubports.com/topic/5373/fairphone-3-fp3-port.
Keep in mind the camera and audio-module do currently not work with this port on the Fairphone 3+.
Good on ya for using Ubuntu Touch!