Hum, well, I understand the semantical difference between boot and flash … but it did not work for me… or so I think.
When doing fastboot boot RECOVERY-FILENAME.img, it just booted the stock android.
Are you talking about installing LineageOS to replace a stock android, or about installing a new version of LineageOS to replace an already installed LineageOS ?
Both. I never installed twrp on my FP3. It was only on the FP2. I followed the recommendation to simply not install it as it’s so easy to just boot it transiently…
Hey, I succeeded in building FP3 zip from source today! 
So far the good news, but many questions arose during this process.
I followed the instructions on the lineageOS page for FP3 and found it helpful but not perfect.
E.g. I had to use the process described in ‘Extracting proprietary blobs from payload-based OTAs’ and could not mount one of the three images, because it did not exist. So I just mounted two images and crossed fingers.
Second, the target ‘FP3’ is not listet in ‘breakfast’, but three other targets are. I nevertheless tried with ‘breakfast FP3’ and ‘brunch FP3’ . But now I dont know if I have a ‘user’ build, with or without ‘debug’.
The whole thing took about five tries to succeed and more than one day (downloading and everything).
During compiling with ‘brunch FP3’ once there was an clang++ error, once some kernel build missed a file, once some strange java error etc.
So this leaves a slight ‘unstable feeling’ about the process and about this ninja tool used to build FP3.
But after ~5 tries I now have this file ‘out/target/product/FP3/lineage-17.1-20210210-UNOFFICIAL-FP3.zip’ and wonder what to do.
On my FP3+ I have installed the beta OTA zip provided by dk1978 from last december.
Can I just sideload my zip the same way without loosing data?
Do I have built an OTA Image now?
Now that I done it I have to understand better WHAT I did 
So that I can adapt the process to my needs, e.g. build my own software for it.
Thanks for hints and for the work done by everybody here!
Hi Guys,
Thank you for getting this to where it is, it’s really great.
How can tip/gift the developers?
See also the discussion here. Bottom line: the Fairphone 3 has some real security issues which are independent of the OS you are running (LineageOS, Fairphone OS, /e/). On the other hand, it makes the phone very re-usable, as it is virtually unbrickable 
To get a user build, use breakfast FP3 user. The default breakfast FP3 produces a userdebug build.
Thank you for that clarification. After having read all facts, i am chilling down for now. I consider the phone as enough secure for my needs and a stolen phone could only be wiped for reusing, as far as i got it. That seems ok to me. Knowing the advanced methods of accessing the device when bricked is helpful, too for the future. Very interesting!
How would one achieve your level of security “re-locked bootloader with additional security measures” - By chance is there a guide on this? Or would you be willing to name those addiotnal measure so we can all benefit?
Thank you
Thank you.
I have to read more about this google / android build process.
Maybe I find a forum to discuss questions.
got some more information for the video conf crasshing-issue at LOS:
It looks like some media related processes die and can’t get restarted
Sure!
In summary, here is what I did:
Sign with my own keys (instead of the google test keys): see the official LOS guidance.
Enable verity mode: comment out following line in device/fairphone/FP3/BoardConfig.mk:
BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS += --set_hashtree_disabled_flag
Create a ‘user’ build (instead of ‘userdebug’): use breakfast FP3 user instead of the default breakfast FP3
Let me know if any questions.
Hey Guys,
I’m stuck here…Has anybody done this on Mac
( OEM unlocked, device state unlocked on the restart bootloader menu on screen, connected to mac )
xxxxx:platform-tools imac$ ./fastboot devices
Axxxxxxxxx fastboot
ziparchive W 02-13 14:55:56 36481 2197659 Unable to open ‘lineage-17.1-20210208-recovery-FP3.img’: No such file or directory
fastboot: error: failed to open zip file ‘lineage-17.1-20210208-recovery-FP3.img’: I/O error
or I get this…
xxxxx:platform-tools imac$ ./fastboot update lineage-17.1-20210208-recovery-FP3.img
fastboot: error: failed to open zip file ‘lineage-17.1-20210208-recovery-FP3.img’: Invalid file
I’m following this ✏ [ALPHA] FP3/FP3+: Complete guide for installing LineageOS I’m stuck on stage 5 as per above.
I think I’m going wrong here…
As I have the img on my desktop and I don’t know where the equivalent of the destination above is for OSX.
Any help would be appreciated.
Thank you
You should copy the file from your desktop to the folder where you run your adb commands, so where ‘platform-tools’ is located.
Thank you - It’s already there…Still nadda. Please see above
xxxxx:platform-tools imac$ ./fastboot update lineage-17.1-20210208-recovery-FP3.img
fastboot: error: failed to open zip file ‘lineage-17.1-20210208-recovery-FP3.img’: Invalid file
I just quit out of terminal and reestablish the directory and it worked partially but I got this…
XXXX:platform-tools imac$ ./fastboot flash recovery lineage-17.1-20210208-recovery-FP3.img
Sending ‘recovery’ (65536 KB) OKAY [ 1.706s]
Writing ‘recovery’ FAILED (remote: ‘partition table doesn’t exist’)
fastboot: error: Command failed
Maybe the alpha documentation isn’t up to par any longer… Might be easier to follow the instructions from the LOS wiki (in doubt see post # 1)
Where did you get this command from? I don’t see it in the guide you linked and it is just wrong.
check the file for corruptions. i had the same issues on another device and the hint on that install guide was to rename the image file to something short, like recovery.img.
Good Evening,
The line you are questioning was my attempt at improvisation as I could not get the Alpha or offical Wiki guide to work.
I’ve gone back to the offical Wiki which is here Install LineageOS on FP3 | LineageOS Wiki and I’m stuck here.
See attempts below.
XXX:platform-tools imac$ fastboot devices
-bash: fastboot: command not found
XXX:platform-tools imac$ ./fastboot devices
AXXXXXXXXX fastboot
XXX:platform-tools imac$ ./fastboot flash boot recovery_lineage-17.1-20210208-recovery-FP3.img
fastboot: error: cannot load ‘recovery_lineage-17.1-20210208-recovery-FP3.img’: No such file or directory
XXX:platform-tools imac$ ./fastboot flash boot lineage-17.1-20210208-recovery-FP3.img
fastboot: error: Couldn’t parse partition size ‘0x’.
