Fairphone 3

Wikipedia has a page about Fairphone 3.

LineageOS

LineageOS has a page with good info about FP3 (Fairphone 3(+)).

Bypass Screen Lock

See "How to Bypass Screen Lock on FAIRPHONE 3 - Hard Reset by Recovery Mode / Delete All Personal Day".

Unlock/lock bootloader

About (un)locking bootloader of Fairphone 3(+) there is a good Fairphone article: "FP3(+). Manage the bootloader".

Unlocking the bootloader only needs to be done once per device.

To unlock bootloader device has to be online. If bootloader is already unlocked and device is offline, it is possible you see after enable the Developer Options by going to Settings → About phone, pressing seven times Build number (a message appears telling that Developer mode is enabled), going to Settings → System → Advanced → Developer options and scrolling down until OEM unlocking: "Connect to the internet or your carrier".

Just connect to the internet, use app switch and "CLEAR ALL" before verifying status of OEM unlocking.