As promised TWRP 3.4.0 has just been released:
https://twrp.me/site/update/2020/06/24/twrp-3.4.0-0-released.html
And with its release FP3 is now officially supported:
https://twrp.me/fairphone/fairphone3.html
Unfortunately for some reason I am unable to edit the OP (It just shows me the post history)
If anyone knows what’s up with that?