First, complete all the steps from the article Fairphone 3(+) OS manual installation.

Apple macOS X 10.1x process

Once your Fairphone 3(+) is in Fastboot mode, connect it to your computer using a USB-C certified data cable.
Locate the archive you downloaded in the previous step, (usually in the Downloads folder). Open Chrome Browser.
1. Press simultaneously the keys Alt ⌥ (option) + Cmd ⌘ + L. A new tab called Downloads will open.
2. Look for the last download, the name starts with FP3-.... Click on Show in Finder.
  
  A new instance of Finder will open.
Open (Double-click) the archive you downloaded, the name starts with FP3-.... The files will extract in a folder with the same name. Wait for it to be completed.
Once done, press simultaneously the Cmd ⌘ + Space keys. Spotlight Search will open.
Write Terminal.app, then press the Enter ↩ key.
In the Terminal window, type cd then press space.
Drag the folder extracted previously inside the terminal window, and press the Enter ↩ key.
Type ./flash_fp3.command (alternatively ./FP3_FlashAll.command) and press the Enter ↩ key.If a similar error message appears:
1. Click on Cancel.
2. Go to the Apple menu (top left corner of the screen) →System preferences....
3. Click on the icon Security & Privacy.
4. Click on the General tab.
5. Click on Allow Anyway.
6. Now, go back to your Terminal window.
Type ./flash_fp3.command and press the Enter ↩ key.
The script will now run for a couple of minutes like in the following picture.
Once done, you will see the following picture.

-IMPORTANT-
It can take up to 15 min. Do NOT interrupt the process, it might damage the device!
Done, your phone is now up to date. Your phone will now reboot, finishing the system update for a few minutes. You can now safely unplug your Fairphone 3(+) from your computer.
Once done, for safety reasons, we strongly advise you to lock again your bootloader. To do so, follow the instructions here.