Fastboot needs to be updated. 1.43 minimal adb and fastboot will not work. Downloaded and extract the Standalone Android SDK platform.
Next put the Graphene OS files in the Standalone folder, you can rename the folder whatever you want.
Open cmd or powershell. Note: commands are different for powershell.. I'll be explaining using cmd.
Type adb devices, the device should be listed.
Adb reboot bootloader
Fastboot devices, it should be showing.
Type whatever the name of the flash all file is, nah lets make it easy drag the flash file into cmd and press enter.
Wait and watch cmd.... You shouldn't see any errors and the Pixel should show Graphene OS fastbootd
Oh if you want to know if you're in fastbootd prior to this type fastboot devices,
fastboot getvar is-userspace it should say yes.
Anyways after it shows Graphene OS fastbootd it'll run the script and reboot into Graphene OS.
After your Pixel boots, you'll need to boot into fastboot and type fastboot flashing lock then reboot.
Note: The bootloader needs to be unlocked prior to installing Graphene OS and relocked after it boots.
OEM unlocking can also be turned off AFTER flashing and successfully booting Graphene OS.
Samaco's Pixel 3a XL now has Graphene OS installed. I connected remotely and installed it.
A few things i left out like verifying the file etc. That's up to you, i use my own methods.