Do a full unbrick method 1 here : https://forums.oneplus.com/threads/guide-mega-unbrick-guide-for-a-hard-bricked-oneplus-3.452634/
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.