Riding out the summer doldrums with a guest-curated group show is standard practice for name-brand galleries. Handing over the exhibition-making reins to an outsider—preferably a bleeding-edge tastemaker—allows for some quirky deviation from familiar programming. And the game of mix-and-match can, when the chemistry is there, cast selector and selections in a revealing new light. Three current examples of this appealingly unpredictable subgenre—all organized by men-about-the-not-for-profit-art-world for established Chelsea powerhouses—represent divergent approaches to the task. But while varied in their ambitions, all set an easy-going tone—too high-minded to be trashy beach reads, they’re still page-turners.