That's not 100% accurate. It depends on what kind of motivation those people have. Say they are professional sportsmen. Then, it is reasonable to believe that 3 men are 300% effective. Say they are just schoolchildren killing their time on PE classes. Then, they'll not be at 300%.
What we are primarily talking about is collective responsibility or whatever it's English name (I never learned physcholoy in English). The more people take part in an activity the less responsibility one must take if the activity's outcome is unfavorable, say they are defeated in football.
Still I cannot see how that explains the stormtrooper effect. All we are dealing with here is that the heroes must survive against impossible odds in the movies, and not only the Star Wars movies, but movies like Rambo or any other conventional action movies. I mean, why would allegedly highly trained troopers disturb each other so much as to decrease their effectiveness so drastically?
