The “3rd” might just be a deliberate false witness or two...
Just to mess with the paranoid folks who think everything is a ‘conspiracy’
I have no idea. But I do know when the terrorist entered that building in San Berdo and started shooting folks to death while they were enjoying their Christmas party, it was total nightmare of chaos, hysteria and total confusion. Fog of war if you will. Btw, you can bet LE already knows if there was a 3rd shooter in that building.
I personally believe there are likely hundreds or maybe thousands of these Muslim cells, individuals or groups just waiting for their own missions.