Unlock Bootloader from EDL mode via MSMDownloadTool
Here’s what anyone should do if it’s in a similar situation:

Do a full unbrick method 1 from here

Download OxygenOS 4.0.0 (Android 7.0) , OnePlus3Oxygen_16_OTA_035_all_1612310359_e10cadfb2af7.zip.

adb sideload the file above when in recovery. Why this version? Because it’s vulnerable to CVE-2017-5626 and CVE-2017-5624.
    Weirdly enough, my phone turned back from the dead after this; however if it didn’t work (as I was expecting) , I would go into fastboot mode and unlock the bootloader from there with:

fastboot oem 4F500301

Which will completely bypass the oem mechanism and will unlock your bootloader even if you can’t boot the system and go to the developer menu.

This backdoor was removed by OnePlus with OxygenOS 4.0.2 (“patched”).