In the previous month, Google had launched the controversial Pixel 3 and Pixel 3 XL. Pixel phones are believed to be easily customizable phones as they come preloaded with stock Android and on top of that flashing of the phone is quite easy using fastboot and the bootloader is unlockable without breaking a sweat. Moreover, XDA forums can be found full with customized ROMs, kernels, and TWRP, however, these mods need a custom recovery.
Speaking of which, TWRP is now available for the latest Pixel phones. According to TWRP website, TWRP is preferred custom recovery for most of the popular devices. The founder of TWRP (Dees_Troy) reported today about its port for Pixel 3 and Pixel 3 XL.
TWRP flashing is not that easy as compared to that of fastboot flash because Pixel phones have A/B partitions. In order to complete flashing, one needs to boot TWRP using fastboot image and then flash the ZIP file through booted TWRP.
However, in case if you need any assistance, you can always look up this link for instructions. Developers can be persuaded to create more customized ROMs, kernel and recoveries for the device if they are backed with TWRP.