One major distinction is that mirroring consumes two screens. Mirroring means whatever is on one screen is showing on the second screen, therefore you can't use the first screen to do other things without interrupting the second screen. Casting does not do this.
I have a PC with multiple monitors. I can use Chrome Cast Tab to Chromecast on one monitor full screen and run on the other monitor other computer applications at the same exact time while the monitor contents are being cast.
Like he said, “Mirroring” uses two screens. The screen being mirrored, and the screen it is being mirrored onto. Your setup has three screens, of which two with identical content, are taking part in the mirroring.
The local source content on one screen is being streamed from that local screen to also appear on another screen. If the local source content changes, or if the device is powered down, then either the content of the other screen changes also, or the mirroring halts.
“Casting” takes the remote source of one screen, ‘casting’ the content from that remote source, to another screen, leaving the original screen free to do whatever one wants. The other screen continues to display that remote source. With your setup, “casting” will leave you three screens with independent contents. The device which originally had the content can even be powered down, and the screen which received the cast, will still display the cast.