Ok, let’s try …
Step 0.
Don’t really install TWRP if you want to keep your Fairphone OS updating automatically. Installing TWRP means changing the boot partition, which will break OTA updates of Fairphone OS because the updating process checks whether the partitions are unchanged.
This is now different from how TWRP and OTA updates worked on the Fairphone 2.
Step 1.
Unlock the bootloader. This will also do a factory reset.
Heed the warning …
Step 2.
Instead of installing TWRP, just boot it with fastboot from a computer (fastboot is part of the Android SDK Platform Tools) …