Shoot the first one with a gun nearest to you. If the other keep coming, shoot the loudest or closest........
Rob Pincus put out a pretty good video in which he addressed the multiple assailant scenario. It's called "Shooting in Realistic Environments" which fits in quite well with the subject under discussion.
I'm in complete agreement with him that your mind can't process all that information that fast. You aren't going to figure out that Bubba #1 has a knife and he's closest, but Bubba #2 has a shotgun 15 ft. away, and Bubba #3 has a handgun and he's behind Bubba#1. I like his strategy to start engaging as best you can and fight--with cover and lateral movement being potential life savers.
If you happen to see the guy with the shotgun and decide he's the biggest threast and want to engage him first while Bubba #1 is about to slice your liver in two with his knife, or #3 is about to perforate your frame with 9mm, then good luck to you.
That doesn't mean that one or the other won't be the obvious biggest threat and allow you to act accordingly in some instances.
If you were standing on the edge of a small stream and some one gave you a push, you be dancing across without time to decide what rock you're going to step on ahead of time. You'll just be doing it.
Just my thoughts on the matter.