Since the very first Nexus devices, Google's phones have always been great for modding. If you like custom ROMs or rooting, every Nexus and Pixel phone has been easily unlockable. It's now a bit easier to install custom system software on the Pixel 3 and 3 XL, thanks to an official release of TWRP.
TeamWin Recovery Project, or TWRP for short, is a replacement recovery partition developed by the Android community. It lets you install custom ROMs, root your phone, perform a full backup, and more. Official ports for the Pixel 3 (blueline) and Pixel 3 XL (crosshatch) are now available, but there are a few caveats — MTP and ADB does not currently work. If you need to transfer files, you'll need to connect a flash drive to the Type-C port.
You can download TWRP for the Pixel 3 and 3 XL from the source links below. You can also install it using the official TWRP app.