[Google Pixel 3 Guides, News, & Discussion] rczrider:
There's a ton of information out there, but finding it can sometimes be a challenge, especially for a newbie. And then there's posts to read through, in case a process has changed and it's only talked about in a single place, buried any number of pages back. So this is my attempt to put all Pixel 3 how-to's into a single thread. Everything below is what worked for me, from start to finish, with a fresh phone directly from Google that was bootloader-locked, unrooted, and running stock Android. Comments are welcome and encouraged: if something doesn't work for you, or you have a better/quicker/more reliable way of doing something, or maybe I made a mistake...let me know!

A few things:

Everything in this guide was tested on a "regular" Pixel 3 (blueline). It should all work exactly the same on a Pixel 3 XL (crosshatch), as long as you download the appropriate files (specifically the factory/OTA images and TWRP). I'm posting this in both blueline and crosshatch forums.

This first post will have links to files and the tools you need, as well as general things (like how to enable debugging and boot into recovery). The second post has how-to's for the big stuff (flashing images, TWRP, unlocking, etc).

I'm not responsible for anything you do. Follow these guides at your own risk. Doing so may also void your warranty.

Files and URLs

Official Android files

Factory (wipes device and requires unlocked bootloader): https://developers.google.com/android/images

Full OTA (leaves data intact, can be done via ADB sideload): https://developers.google.com/android/ota

Preview/beta OTA: https://developer.android.com/preview/download-ota.html

TWRP

XDA thread: https://xdaforums.com/pixel-3/development/twrp-3-2-3-0-pixel-3-t3861622

Download: https://dl.twrp.me/blueline/

Magisk

XDA thread: https://xdaforums.com/apps/magisk/official-magisk-v7-universal-systemless-t3473445

GitHub: https://github.com/topjohnwu/Magisk/releases

ADB and Fastboot

Google USB drivers: https://developer.android.com/studio/run/win-usb

SDK platform tools: https://developer.android.com/studio/releases/platform-tools

Download the ZIP and open (no need to extract the whole thing)

Copy the files below to their own directory (you will use this directory for all ADB/Fastboot commands):

adb.exe

AdbWinApi.dll

AdbWinUsbApi.dll

fastboot.exe

General

Enable USB debugging

Download Google USB drivers (see above)

Download SDK platform tools and copy required files to their own directory (see above)

Enable USB debugging (Settings > About > tap Build 7 times > back > Developer options)

Plug phone into computer and when "Allow USB debugging" box comes up, check the box and allow (may need to have adb daemon running on the computer already, see below)

Fastboot mode (using hardware keys) aka bootloader

Power off

Hold VOL- and press/hold PWR until bootloader (fastboot) comes up

Recovery mode (using hardware keys)

Get into Fastboot mode (see above)

Use VOL buttons to find Recovery in the list, use PWR to select (should boot to broken android image)

Hold PWR and press VOL+ once to get into recovery

Useful ADB and Fastboot commands

Enable USB debugging (see above)

ADB (used while phone is "on" and booted up):

Use "adb devices" to ensure the computer is authorized

Use "adb reboot bootloader" to get into Fastboot

Fastboot mode

Use "fastboot devices" to verify computer can see your phone