It’s a tidy fantasy that artists birth masterworks in a single assured gesture. It’s equally illusory to think that there are fixed ways for a museum to present an artist’s output. Three thematically related fall exhibitions at the Cantor Art Center explore the various spheres of influence and process in order to illuminate the complexities of making and appreciating art. “Edward Hopper: New York Corner,” “Richard Diebenkorn: The Sketchbooks Revealed,” and “Artists at Work” are autonomous shows that together open up inspiring new angles from which to view familiar artists.