I use 3 2 1 for my workflow:

I keep my photos on a mirrored NAS but store my actual catalog on an SSD. Lightroom generates and caches previews of photos and stores them alongside the catalog on the SSD in this workflow.

Lightroom is designed to accomodate photo storage being attached and detached. You can cache previews on the SSD and work with them without the main storage being attached.

You could also have multiple external devices for storage and Lightroom will just show the path as missing until reconnected.

This accomodates a workflow where perhaps you have a laptop with the catalog and your photos are stored on an external USB HDD. You can take your laptop away to do editing and reattach the original photos for final rendering to jpg.

I use crashplan to keep my photos and catalog backed up, but Lightroom can also be configured to copy photos to two places during imports. The feature is called "Make a Second Copy to".

The copies won't be included in the catalog metadata, but it is a way to keep the 2nd copies stored somewhere, but unmanaged by Lightroom. Some people use this method to have a backup immediately on a multiple devices while the traditional backup strategy works in the background (like a cloud based offsite backup.) Some people also don't reformat their camera storage card until they are sure backup processes have completed.

I also turn on XMP sidecars to store photo editing metadata alongside the original photos just in case my catalog were to get lost or corrupted.

(You'll notice the Lightroom catalog isn't exactly described as 3 2 1, but I do keep it on a Dropbox synced to multiple workstations...)

This answer only covers Lightroom Classic - Lightroom also has a new cloud version where your photos are stored online. I'm not as familiar with that version.