Backup Pixel 3 before erasing phone with TWRP

I am attempting to take a recovery/backup of a Pixel 3 using TWRP. I do not need to flash TWRP, just boot from it to perform the backup. Meaning, I don't care if TWRP is installed permanently. The problem I am running into is that I need to unlock the bootloader before I can boot TWRP. And if I unlock the bootloader, the data on the device will be erased. **My question is:** If I unlock the bootloader, will I be able to boot back into fastboot mode, and thus boot into TWRP, before the erasure occurs? And if to, how do I perform this? (I presume that if I hold the volume down button it will before rebooting it will go back into fastboot mode.) *I am trying to obtain a backup of the phone before the erase script is activated.* I have searched around and have not been able to resolve this question as of yet. Thanks in advance for any assistance provided.