To open fastboot mode on the Asus ROG Phone 3, you can use a physical button combination or an ADB command from a computer. Fastboot mode (also known as Bootloader mode) is a protocol used to flash firmware, unlock the bootloader, or perform deep system repairs when the device cannot boot into Android. Method 1: Physical Button Combination

[Phone] How to do hardware reset on your phone | Official Support - ASUS

Execute Command: Open a command prompt or terminal on your PC in the folder where your ADB tools are located and type:adb reboot bootloader

Method 2: Using Hardware Keys

1. Power Off Device: Power off your ASUS ROG Phone 3.
2. Press and Hold Volume Down and Power Buttons: Press and hold the Volume Down and Power buttons simultaneously for 5-7 seconds.
3. Release Power Button: Release the Power button but keep holding the Volume Down button.
4. Fastboot Mode: Your device will enter Fastboot mode.