Pixel devices are known to be one of the most developer-friendly smartphones on the market now. They come with the “stock” Android experience, the bootloader is easily unlockable (at least on the international version), and flashing anything via fastboot is no hassle. That helps the always growing developer community evolve even more. XDA Forums are full of custom ROMs, kernels, and custom recoveries like TWRP. But these mods need to be installed on the device via custom recovery. The latest representatives of the series, the Pixel 3 and Pixel 3 XL, were released barely a month ago. Both of the devices feature Snapdragon 845, 4GB RAM, 64/128GB storage, wireless charging, IP68 water and dust resistance and other features.
The most known custom recovery for this time is TWRP. It has official support for a lot of devices, as you can see on the official website. Today, the founder Dees_Troy announced official TWRP support for the Pixel 3 and Pixel 3 XL, so you can start flashing anything you want. Unsurprisingly, in order to flash anything via fastboot, even TWRP recovery, you first have to unlock the bootloader. The instructions for the procedure are available at this link. Due to the fact that Pixel devices use A/B partitions, TWRP flashing procedure is ever so slightly more complicated than your typical ‘fastboot flash’ command. You have to boot TWRP via fastboot image first, and then flash the ZIP through booted TWRP.
Detailed instructions are available at the links below for the respective devices. Official support of TWRP recovery will push the developer community to develop even more custom ROMs, kernels, and recoveries for the devices. You can see all of that on our forums for Pixel 3 and Pixel 3 XL. Forum threads for TWRP recovery on both of the devices can be found below.