I think the chances of finding 3 psychopathic non-hispanic white guys who would be able to coordinate plans for a mass homicide and not have one inform authorities about the other two in a primarily black/hispanic town are fairly low.
We can probably exclude jihadists because they couldn't find San Bernardino on a map. That leaves drug gang violence or "workplace violence". (Or the "3 white males dressed in military gear" were part of the SWAT team.)
Maybe a Nazi gang but I doubt that.
Lack of a BOLO tells me that they think the gunman is inside.
Great point! Funny how the first stories to come out all fit the profile.