Please try this perl script with your IMEI (slot 1) and serial number:
perl fp3verifycode.pl --imei 123456789012345 --serial A1234567890
(The numbers can be found under “About phone” - “Model & hardware”)
This should print a string with 8 hex chars at the end for the verify code.
You might also check adb logcat when trying this, maybe there is some output from the OemUnlock code …
I cannot test it myself as I have unlocked mine directly after receiving it and before installing any updates.
Yes, that worked for me too! Thank you so much, you’re brilliant!
Huh, where did you get that algorithm from?
Looking at Settings.apk which I pulled from my Fairphone 3.
The check was probably added by Fairphone with one of the updates as the "WARNING: Device protection… " is a normal AOSP string but “Input verify code :” does not exist anywhere else.
Neat! From the dexdump ? 
There should be a way to like a comment more than once.
I’ve put up the algorithm on my website so people don’t have to run perl locally 
Full credit belongs to @_tmp!
https://public.z3ntu.xyz/FP3/verify-code.php
Funnily I didn’t have to change much from the original code because Perl and PHP seem to provide the same functions and the rest of the syntax is also more or less the same 
And now thinking about this, it would have probably been nicer to implement it as client-side JavaScript. Oh well.
nice. I noticed, it’s missing the length/validity checks for imei and serial number’s though, so if you ever do a javascript reimplementation, you might like to implement those, too
Correct, I ignored them for now. Will implement that later though
It works for me. Thank you.
Add a length check now, I’ve also uploaded the script running on my server to gist as well: https://gist.github.com/z3ntu/9c09120bb44c6dd9fe163931a1478ea9
Apparently Fairphone published their own How-To for unlocking the bootloader 
EDIT:
there are some concerns regarding the data which is transmitted to Fairphone: keep in mind that fairphone may save your device informations (IMEI & SN) and thus Fairphone knows wheter you unlocked your device or not (could be important in warranty claims!)
Alternatively, you can use this method
I would untag the previous post as “solution”, and rather redirect any interested person to ✏ FP3: unlock the bootloader or to https://private.z3ntu.xyz/FP3/verify-code.php.
These are the facts:
I have not checked whether the IMEI and serial numbers are actually sent to FP servers when you use their tool or whether the decoding is done client side, it would be interesting to see. (Edit: it has been checked in a later post by justlurking that this data is indeed processed by a remote server) But I let you draw your conclusions.
If I brick my phone, I will consider myself the only responsible for it, and will not open a warranty case with FP. But if there is a hardware failure, I wish FP to respond for it, independently of whether I have at some point intended or not to unlock my bootloader.
Moreover, it is not FP’s business to know whether or not I have the intention to unlock my bootloader. Well, I really don’t know (yet) what the FP tool for getting the code for unlocking the bootloader actually does, but in order to keep on the safe side, I prefer the strategy suggested on the above cited links.
Edit: it seems that there are actually quite a few guarantee-unrelated good reasons for which FP might want to collect, or even might have to collect this data. But using an alternative method to get the unlocking code still does not hurt 
.
It was me who retagged the solution to the official Fairphone solution.
My rationale being … it is the official Fairphone solution to the topic at hand.
But I have no emotional attachment to this detail here.
I have added the official code generator link to ✏ FP3: unlock the bootloader, so anybody with the permission please feel free to retag the solution if you so choose.
I agree, that it is not FP’s business, if you want to unlock your bootloader.
They - obviously - might save a list of those phones, that they sebt unlock-codes for. (I bet, they do.)
On the other hand:
When reading the source-code of this FP homepage, line 23 reads:
So, it seems, there are phones, that can not be unlocked. Possibly those, they sold to companies as business-phones?
I have no idea, if the solution by @z3ntu enables unlocking for those phones as well? But you might be in trouble (with your company), if you unlock such a phone.
Well, alternatively, HolosericaCaligo could edit his/her post by adding a reminder to the links to the alternative way, so that anybody redirected directly to the solution could make his/her own choice.
@BertG: you are right, that could be the reason for FP to proceed in that way. I am not fully convinced FP necessarily keeps track of the devices that have potentially been unlocked, but since the whole process is not completely transparent but there is a transparent solution proposed instead, well, it looks fair that this solution should also appear here well visible.
It does send the IMEI and serial to fairphone; there is a POST call to
https://factory.fairphone.com/api/unlock-codes/qwerty/asdf
where “qwerty” is the IMEI and “asdf” is the serial.
Not sure what Fairphone is doing with this information; hopefully they’re using it to track interest in alternative OSes and to see if it’s worth investing resources.
Agreed. But on the other hand: if somebody is new to the community and just searching for the info how to unlock his FP3, would they rather enter the data to fairphone.com or to a totally unknown z3ntu.xyz? One needs to follow the community a while to find that z3ntu seems to be well-accepted member, whom the community trusts. So, from a security point of view it might be preferable to enter that data to a “official” site. Therefore, if that code is needed, I think it makes sense, that Fairphone offers a way to get it. But of course that doesn’t answer the question why the code is needed and what Fairphone does with the numbers.
Maybe they included the “input verify code” box due to legal reasons. As already posted in other topics they have to pay a fee for every device with an unlocked bootloader. Instead of paying this fee for all devices they could have implemented this verify code to pay for unlocked devices only. As each (official) unlock attempt requires entering the serial number and imei at their unlock website, they are able to track the state (locked/unlocked) of the bootloader.