Johnny Knoxville and company return with another collection of stunts, pranks, and candid-camera gags. The Jackass movies are in a class by themselves, but now that we have three (in addition to the short-lived MTV series), one can safely say that 3D is the least inspired. There are a few Keatonesque gems (an obstacle course of dangling stun guns and cattle prods, a game of tetherball with a beehive for the ball), but ingenuity mostly gives way to scatology (the “Poo Cocktail,” in which a portable toilet with a full load is launched into the air with someone inside). At their best, the Jackasses combine low-brow humor with delectable absurdity (one of my favorite gags from Jackass: The Movie had a guy creeping up on a cougar while dressed as a giant mouse), but here it’s almost pure punishment. Weirdly, the end credits roll over a sentimental montage of images from the members’ younger days—as if building the Poo Cocktail represented some sadder maturity. Jeff Tremaine directed. R, 94 min.