Custom ROMs are the saviors of old Android phones. Google's Pixels get quite a lot of after-release support, but with the Pixel 2 series hitting end-of-life back in December, the writing is on the wall for the Pixel 3 and Pixel 3 XL. So if you're ready to say goodbye to official software updates a little early, why not hop on the Lineage train?
Nightly builds of Lineage 18.1 (based on Android 11's open-source code) are now available for both devices, ready to flash at your convenience. Previously the official Lineage builds for the devices topped out at Android 10. Being nightlies, they're unlikely to be entirely stable and might have a few bugs hiding in dark corners. But Lineage nightlies are much more ready for primetime than that term usually implies, especially on well-known Pixel hardware.
To install Lineage, you'll need a custom recovery to flash the file, and you'll probably want a recent build of Gapps to add in the Play Store and other essential Googley services. If you'd prefer to play it safe, you can wait for a "stable" build, but those seem to be rarer and rarer these days.