I don't think a two-shot derringer is adequate for dealing with three attackers
(and no, I don't think two cartridges is reasonably sufficient for even a single Angry Dude, I just went with the funny one)
Actually, it would probably work just fine about 90% of the time.
When I was busy flunking out of Air Force navigator school at Mather AFB my new bride and I lived in an off-base apartment in run-down Rancho Cordova. No carry permits in CA in those days, at least for semi-transient military types, but when I would go for my evening jog I would put my trusty Beretta 950 .25 in my sweat pants pocket.
One evening as I plodded along three yutes on the other side of the street stopped and course corrected to intercept my path. There was no one else around. Maybe they were coming from a dress reheasal for Gangbangers On Ice, or wanted to thank me for my service, or meant to rescue a kitty from a tree behind me. No way to tell.
I said Hi Fellas and held up my little Beretta like I was showing them a pocket watch. Didn't point it at anyone, just making the I Have A Gun point.
They stopped, conferred, and the lead guy grinned and gave me a little wave. We all went on our ways.
Before all the MMQBs pile on with the accumulated wisdom of their CCW Instructors, I'm not recommending this approach to anyone. I know there are lots of people who don't consider their gun a threat management device short of gunfire. Good for them.
In this case nobody got shot. I didn't get robbed or stabbed. For some reason these young folks didn't report a felonious brandishing by a slow moving jogger. I went on to absolutely bust night celestial navigation at which point the USAF made good on their promise to make me a cop and send me to Minot by doing exactly that. I had a ball as an AF cop and then an FBI agent.
My little Beretta wasn't a derringer, but a derringer would have had the same effect.
I don't think a derringer is an optimal tool, even though I know a UC agent who killed a guy stone dead with a High Standard .22 Mag. But ANY gun is better than no gun, because any gun fulfills the I HAVE A GUN role which has defused countless altercations.