You can now unlock bootloader of your Nord 3 by typing just 4 commands. Rooting OnePlus Nord 3 also got easier than ever. All you need is enable developer tools, and OEM unlocking option followed by USB debugging.
You will also need to download and install Google SDK Platform Tools on windows computer. Once you have done that, connect your OnePlus Nord 3 to the PC.
adb devices
adb reboot bootloader
fastboot flashing unlock
fastboot reboot
Your OnePlus Nord 3 should now reboot with an unlocked bootloader. Now, we will be rooting it. For this, we will need the stock boot.img file which is downloadable from here. On the Nord 3, you will need to download and install Magisk Manager. Open Magisk on your Nord 3 and go to install > select and patch a file > Browse for boot.img
The app will now patch boot image of your Nord 3. This file will be outputted in the Download folder. You need to copy this patched file from Nord 3 to your windows computer.
On the computer, fire up command prompt and go to the ADB folder. Type in these commands.
adb reboot bootloader
fastboot flash boot magisk_patched.img
fastboot reboot
That is it, you have successfully rooted OnePlus Nord 3.