[ASUS ROG Phone 3 Guides, News, & Discussion] Invisan:
So i was searching for some guides but none were really complete so here is my take on a Guide regarding Root and flashing from WW to CN or CN to WW and Downgrading if your FW is to high.

Requirements:

A computer

Your ROG Phone 3 (Duh)

The Bootloader Unlock Tool (can be found here)

TWRP (can be found here)

ADB+Drivers (can be found here and here)

The newst Firmware from Asus (can be found here) To cross flash you need both CN and WW Version with the same Build Number. Newest one with matching Number should be the .2009.98 as the time of writing this. CN WW

Magisk Manager (can be found here)

IMPORTANT!

After you cross flash you will loose the ability to update via OTA. So you have to update your Phone yourself.

Also you WILL loose your Data by following this guide and unlocking your Bootloader!

Follow this Guide at your own Risk! I do NOT take any responsibilty for problems or bricks.

Also if you have a PIN, Fingerprint or anything installed on your Phone aside from the PIN for your Cards disable them.

Preparations:

First download or copy the Bootloader Unlock file to your phone and install it. After that run it. If it is successfull your Bootloader will be unlocked and your device will be wiped.

Now install adb and the drivers provided from the Links above.

After you installed adb download and copy the twrp image to the Adb folder and rename it to twrp.img

Then open a cmd and cd to the directory where adb is installed. Easiest folder for accessing + location would be C:\adb

Now enable the Developer Settings on your Rog Phone. For that open the Settings go to System / About the Phone / Softwareinformation / Then tap on Build-Number till it says you are a Developer.

Now go to Developer Options. Its also under System and then Developer Options. Enable USB-Debugging.

Connect your Rog Phone via usb to your pc and enable the ADB Debug Connection from the prompt on your phone.

Type in adb reboot bootloader and your phone will reboot to fastboot.

After that use

fastboot flash recovery_a twrp.img

fastboot flash recovery_b twrp.img

to install TWRP on your Phone. After that reboot your phone to recovery through the menu. With TWRP installed you can now fully Backup your system + install/downgrade new Versions of Android. This also works if you want to install WW on a CN Version of the Rog Phone.

Downgrade:

To Downgrade just download the Firmware Version you want to downgrade to from the Asus Homepage and copy it to your ROG Phone.

Then boot to TWRP. To be safe create a backup. After that you can install the ZIP file with TWRP. If the Downgrade is successfull you can remove the backup to save some space.

Reboot your phone and check the installed Version.

Switching Rom WW/CN or CN/WW:

To Switch your Rom you need the Firmware Version which matches for both Region. Currently this would be build .2009.98 There are newer ones available with 2009.99 for WW and 2009.100 for CN but the later one is not available for Download only through OTA and 99 is not available as download for CN devices.

If your Firmware is higher then the .98 or newest matching Version available Downgrade to that Version. Create a Backup before doing so.

If successfull you can then install the same Version for the other Region. For me this did work fine. If it does not work you can always restore your backup via TWRP.

There are also RAW Zip Files available for some Versions but those are hard to find and can brick your phone while trying to downgrade.

Rooting:

Download and install Magisk Manager from the link provided above.

After installing it open the App and download Magisk.

Boot to TWRP and install the Zip File for Magisk.

After that reboot to System. You can check if root is working by using root checker or any other software.

If you plan on using Magisk together with for example Pokemon Go you need to Hide Magisk in the Settings.

Like Bugs Bunny said. "This should be all folks."

If you need further help or something is missing feel free to post below.