Ya know, if the death of cinema actually happens, it will be because of the youngins' lack of attention span. It might be compounded by the odd death of the intermission in movies.
Seriously though, three hours is nothing if it's a good movie. It's not something to bitch about.
Branagh's Hamlet was over four hours. Gettysburg was over four hours. 1900 was about five hours. Once Upon a Time in America was over four hours and the director's cut was over five hours. The Best of Youth was almost seven hours. The Soviet film adaptation of War and Peace was over seven hours. These are all great movies and entirely worth the time. They're also all worth seeing in the cinema if you can.