step by step :
(not tested on Pixel 3 XL but it might work the same, just use the right firmware for your device)
What do you need :
> Pixel 3 phone with latest Android 10 Build (QP1A.190711.020)
> Unlocked bootloader on your Pixel device
> Latest ADB and FASTBOOT tools on your computer
1 ) Download the latest Android 10 factory image for your device ("Blueline" is the regular Pixel 3) :
Link > 10.0.0 (QP1A.190711.020, Sep 2019)
2 ) Extract the "boot.img" file on your phone
3 ) Download Magisk Manager application on your phone and install it :
Link > can not provide link please go to the Magisk manager website and go to section download.
4 ) Open Magisk Manager app and add the Canary Channel :
> Go to Settings > Update Channel > Custom and then add this URL : https://raw.githubusercontent.com/topjohnwu/magisk_files/master/canary_builds/canary.json
5 ) Restart the Magisk app (close the app from the multi-tasking window and launch it again) and it should now ask you to update Magisk Manager to the Canary channel (very important, this tutorial doesn't work with stable or beta Magisk Manager app. At this date (september 4th 2019) the latest Canary build is 7.3.3-a6126c5e (232)
6 ) Press on the Install Magisk button (the one on the top, not the Magisk Manager), then Install > Select and Patch a File and now select the Boot.img file you extracted earlier. When the process is done, just put the patched boot.img file on your computer in the same folder as your ADB and FASTBOOT tools, the file's name is magisk_patched
7 ) Enable adb debugging on your Pixel phone and connect it to your computer via USB.
Open a command prompt on your computer and migrate to the folder where the ADB and Fastboot tools are located.
(for exemple, for me it's : cd C:\\adb).
8 ) Reboot your phone to fastboot by typing adb reboot bootloader
9 ) Flash the Boot.img you just patched with magisk by typing fastboot flash boot magisk_patched.img
10 ) When i'ts done, reboot by typing fastboot reboot
11 ) Open your Magisk Manager app and chek if everything is good, and it should be ! You can now install Adaway after you enable Systemless root in the Magisk settings and rebooting your phone.