Pixel devices have 2 "slots" for ROMs / firmware. TWRP will detect whichever slot is currently active and use that slot for backup AND restore. There are buttons on the reboot page and under backup -> options to change slots. Changing the active slot will cause TWRP to switch which slot that TWRP is backing up or restoring. You can make a backup of slot A, switch to B, then restore the backup which will restore the backup of A to slot B. Changing the slot in TWRP also tells the bootloader to boot that slot.
The zip install method installs TWRP to both slots.
If you already have TWRP installed: Download the latest zip and install the zip using TWRP.
If you do not already have TWRP installed: Download both the img and the zip. Copy the zip to your device. You will need to have fastboot binaries and the correct drivers installed. Power off your device completely. Hold volume down and turn on the device. Your device should now be in the bootloader. Connect the device to your PC. Open a command window and run the following command from the proper location:
fastboot boot path/to/twrp.img
This will temporarily boot TWRP on your device. If you are using a lockscreen pin/pattern/password and do not get prompted to enter your passord, reboot to the bootloader and try again. Go to install and browse to the zip and install the zip. If you are currently rooted, you will need to reflash the stock boot image before installing TWRP. After installing the stock boot image, follow the instructions for installing TWRP. Once TWRP is installed, you will need to reflash root.
If you accidently flash TWRP to your device using fastboot instead of temporarily booting the image, you will need to download the latest factory image for your device and reflash the boot image.
3.2.3-0: Initial release for the Pixel 3 devices
Pixel 3 XL: