Just wanted to inform you, that rooting with Magisk works like a charm. Even without TWRP.
What you need:
Magisk Manager: https://xdaforums.com/apps/magisk/official-magisk-v7-universal-systemless-t3473445
boot.img from your ROM (prepatched boot.img for CN_V10.0.11.0 ROM here: https://filebase.philipp.technology/filebase/index.php?page=Entry&entryID=19)
Unlocked Bootloader
ADB and Fastboot on your PC (Simple ADB & Fastboot Installer: https://filebase.philipp.technology/filebase/index.php?page=Entry&entryID=5)
Installation:
Install MagiskManager.apk on your Smartphone
---SKIP IF YOU USE THE PREPATCHED BOOT.IMG---
Put the unpatched boot.img on your phone
Open MagiskManager and Click "Install"
Select "Patch boot.img" and choose the boot.img with your file manager
Copy the "patched_boot.img" file on your PC
---CONTINUE HERE---
Now connect your Mix 3 to the PC and boot in fastboot mode (over the command line: adb reboot bootloader)
Navigate with the command line to your folder, where you saved the patched_boot.img
Now flash the patched_boot.img file with fastboot: fastboot flash boot patched_boot.img
If the flash is successfull you can reboot the phone with fastboot reboot
Open the MagiskManager and verify that Magisk is installed and your phone is rooted.
What you need:
Magisk Manager: https://xdaforums.com/apps/magisk/official-magisk-v7-universal-systemless-t3473445
boot.img from your ROM (prepatched boot.img for CN_V10.0.11.0 ROM here: https://filebase.philipp.technology/filebase/index.php?page=Entry&entryID=19)
Unlocked Bootloader
ADB and Fastboot on your PC (Simple ADB & Fastboot Installer: https://filebase.philipp.technology/filebase/index.php?page=Entry&entryID=5)
Installation:
Install MagiskManager.apk on your Smartphone
---SKIP IF YOU USE THE PREPATCHED BOOT.IMG---
Put the unpatched boot.img on your phone
Open MagiskManager and Click "Install"
Select "Patch boot.img" and choose the boot.img with your file manager
Copy the "patched_boot.img" file on your PC
---CONTINUE HERE---
Now connect your Mix 3 to the PC and boot in fastboot mode (over the command line: adb reboot bootloader)
Navigate with the command line to your folder, where you saved the patched_boot.img
Now flash the patched_boot.img file with fastboot: fastboot flash boot patched_boot.img
If the flash is successfull you can reboot the phone with fastboot reboot
Open the MagiskManager and verify that Magisk is installed and your phone is rooted.