This method is the most reliable, provided your phone is booted into Android and responsive.

This is the most common way to enter Fastboot mode when the device is powered off. : Turn off your ROG Phone 3 completely Button Combination : Press and hold the Power button Volume Up button simultaneously. Enter Bootloader

Using Fastboot commands, especially those involving flash or unlock , carries a risk of voiding your warranty and bricking your device if done incorrectly. Always double-check the command before pressing Enter.

Execute the reboot command:

If your bootloader partition is corrupted, the phone will ignore the Fastboot command. This is rare but serious. You would need to use QFIL or ASUS's proprietary flashing tool to restore the bootloader via EDL mode.