Once you have managed to bootloader unlock your FP3 here is output of flashing process:
https://forum.fairphone.com/t/installation-e-sur-fairphone-3/74396/6?u=aspergerguy
You would need to install Homebrew to get fastboot and ADB installed as here:
https://www.androidbeat.com/2015/11/how-to-set-up-adb-and-fastboot-on-mac/