Do I need to root my Pixel 3 phone in order to install GrapheneOS? I am interested in stalling GrapheneOS custom ROM on a Pixel 3 phone.
Does this require that I root the phone?
No, it is neither required, nor recommended to root the device.
You will need to follow the official instructions here: https://grapheneos.org/install
It will require you to:

Upgrade the stock OS (to get any firmware updates)
Show dev options.
Enable OEM unlocking.
Actually unlock the bootloader.
Install GrapheneOS via fastboot
Lock the bootloader.