Nothing squanders built-up goodwill quicker than a third film in a series that’s still making money, but is out of ideas. It’s the “Home Alone 3” effect: audience optimism turns to disappointment, before realizing there are no more stories to tell on this thin construct. “Pitch Perfect 3” isn’t quite that bad, but it’s only a matter of degree. The second sequel to the a cappella choir comedy feels less like a movie than a bunch of deleted scenes strung together in the guise of a plot.