I downloaded SDK Platform Tools for Windows, unzipped it and can execute adb and fast boot.
But the Fairphone, which is visible in the file explorer, cannot be found by adb.
I tried the following:
ads reboot bootloader
error: no devices/emulators found
ads –s SERIALNO reboot bootloader (SERIALNO stands for the correct serial no of the fairphone)
error: device ‘SERIALNO’ not found
ads –d reboot bootloader
error: no devices found
fastboot oem unlock
< waiting for any device >
Any idea how to connect to the device successfully? (already set to developer mode)
Many thanks upfront
Hartmut
Regain your privacy! Adopt /e/ the unGoogled mobile OS and online services
I had to enable Debugging via USB. After having done so adb has run successfully.
THANK YOU!
The fairphone showed up with a green
START headline,
…
‘fast boot mode’ in red and at the end of the text
…
‘device state - locked’ in red
I started fasstboot oem unlock, but this command again was < wainting for any device >
I pressed volume key and power key together, a new RED headline appeared.
After that, the fairphone rebooted.
Any idea how to proceed now?
Thanks a lot! This helped again and the device stated ‘unlocked’ now 
How to get the /e/ OS now onto the fairphone?
Since it’s a Fairphone 3 and the bootloader is unlocked now … by following the install instructions here from “Download /e/ for FP3” on …
https://doc.e.foundation/devices/FP3/install