Disk Utility shows me three hard drives with the same name. How do I find the physical hard drives they refer to?
The method I use is to grab the serial number from the drive. This information is usually printed by the factory on a label on the physical disk so that it is easy to identify.

Start by ensuring you have HomeBrew installed. Then run the following command to install smartmontools:

brew install smartmontools


Then grab the serial number from the failing drive:

smartctl -a /dev/disk2


This assumes that the failing disk is /dev/disk2 - obviously change that to reflect the disk you have ben reported as failing.

In the output you'll find something like:

Serial Number: C8AF902EAB39


(format varies depending on manufacturer)

You should be able to match that up with the label on the drive.