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This procedure will completly WIPE all your data!
This is NOT to root the FP3
FP3 comes with a locked bootloader. This prevents user to write in /boot, /recovery, /system, /modem & other partitions. Unlocking the bootloader removes a layer of protection, but also allows you to flash a custom recovery, a custom ROM, a custom kernel, or a root script.
This will most probably be needed to root your FP3. So if you will need to root your device, I advise you to follow this guide as soon as you get your phone, as this procedure will wipe all your data.
But, I have no idea if this procedure will void the warranty of your phone. It is possible to revert the procedure and go back to a locked bootloader. Once again, you will loose all your data and maybe the warranty of your phone, do this at your own risks and don’t blame me 
Details regarding warranty warning
You will only lose warranty, as the Fairphone Warranty clarifies it, if you cannot revert to FPOS:
i. Damage caused by the installation of a Third Party Operating System. An example of such damage would be if you modify some Android partitions when you flash a Third Party Operating System, which make it impossible to go back to the official Software version provided by Fairphone.
1/ OEM unlocking
The FP3 is giving you the option to manually unlock your bootloader. This is the so-called OEM lock (see at the end for more information).
The first step is then to performed the OEM unlock.
Go to Setting, About phone, and hit 7 times the last entries, Build number, then enter your password. This allows you to access to a new setting menu, Developer options, in System.
So, go to Setting, System, Advanced, Developer options.
- Then, select
USB debugging. - Also select
OEM unlocking.
If you get an additional request to “input verify code”, here’s Fairphone’s code generator … Bootloader Unlocking Code for Fairphone - Fairphone.
Alternatively you can try generating the required code yourself with the perl script described here: Oem unlock "input verify code" - #11 by _tmp (alternatively you can use this website that runs the script in the background: https://public.z3ntu.xyz/FP3/verify-code.php )
First step completed, you can restart your phone.
2/ Bootloader unlocking
For this second part, you will need functionals adb and fastboot. You can download them from here (see at the end for more information).
Plug your phone to your computer. On your phone, accept the connection and tick to always accept this device.
On your computer, open a terminal and test the connection:
adb devices
This should answer something like this:
List of devices attached
A208F7120204 device
Then, you can reboot in the bootloader:
adb reboot bootloader
On the bootloader, you can see the last line: DEVICE STATE - LOCKED
So know, you just have to enter:
fastboot oem unlock
(on Linux you may have to enter sudo fastboot oem unlock)
Then, on your phone, navigate with Volume + / Volume - to accept (power button). This will completely wipe your phone and at the end of the process (it will restart 2-3 times), your phone will have the bootloader unlocked.
3/ Relock the bootloader
Do the same procedure, but with:
fastboot oem lock
Again, this will wipe you phone.