Success, after some dancing with USB drivers on Win10, the flashing did work out. Gee, windows is so bitchy with drivers 
Does anyone still have the original installer from Fairphone for the latest Android 10? According to
https://support.fairphone.com/hc/en-us/articles/360048139032#h_01GMTQS3F73GMK26E0RFAQ2SJE
that would be 8901.3.A.0136
Thanks 
Here?
Thank you! Alas, it contains only executables for Windows…
I am starting to wonder if the releases back then did have linux scripts at all? 
Other than that, for anyone interested: Yes, it does work but I find it somewhat laggy in general. Is it worth it? Probably not 
Still, nice to see other attempts at cell phones, looking forward to the next version 
PS: And from now on I keep my installers in the backup loop 
Some lovable people at Fairphone did consider it and added older final versions:
Awesome! 
PS: Grab it while you can 
Ubuntu Touch “Focal fossa” 20.04 is now available also for FP3 
As there are no stock ROM updates to beta test anymore, I’ve installed Ubuntu Touch on my FP3.
I went for 24.04 stable, not sure if that was a wise choice, just picked the latest stable one.
The list of things that are supposed to work is pretty impressive: Fairphone 3 and 3+ • Ubuntu Touch • Linux Phone
But then, this is nowhere near as smooth an experience as with Android (which I knew already), I just was a bit curious. And it might come handy at the next Hamburg meetup, alternate OSes always seem of interest.
Edit: switched to the 24.04 daily channel, seems like 24.04 is still kind of in development, at least the fingerprint reader wasn’t working on stable but does work on daily.
There are a couple of more glitches: login to the UBports forum doesn’t work for me in Morph browser but some other, chromium based browser. Similarly, setting up an owncloud account fails on login while the same credentials work in a browser session.