Hi everyone !
I’ve installed /e/ OS on a Fairphone 3, with the help of the Easy Installer for Linux, but it was not so easy… It took me maybe three hours, because :
. In french, you don’t have to search for « débogage Android » but for « débogage USB ».
. You have to download and install the last Android’s version (with Google) before your Fairphone allow you to unlock the bootloader (I didn’t understand that immediatly).
. Awkwardly, my phone first didn’t want to accept the manipulation to reboot and install the /e/ OS. I was very scared during several minutes, because it didn’t want to start at all. I had to remove the battery and reput it in to start the phone… with Android’s Google. The manipulation showed on Easy Installer worked finally after a long time of attempts.
Finally I did it ! 
Good luck to all !