There’s a mildly amusing Pompeii gag midway through “Night at the Museum: Secret of the Tomb.” It involves a scale model toy Roman soldier Steve Coogan and his Old West cowboy pal (Owen Wilson) and a monkey in need of extinguishing a model volcano’s fire.

Later, there’s a funny bit of business with Ben Kingsley, playing a pharaoh brought back to life, riffing with Ben Stiller as security guard Larry Daley, about “Exodus” – the Old Testament event, not the new movie co-starring Ben Kingsley but not Ben Stiller.

And then there’s a gag that tops both those rare highlights, one which involves a famous song-and-dance man, a London theater and a wax figure of Sir Lancelot (Dan Stevens of “Downton Abbey”) who is dashing and yet awfully confused upon coming to life in The British Museum.

Otherwise, “Secret of the Tomb” is a tedious, sentimental affair, a kid-friendly comedy with delusions of “Toy Story 3” when “Ace Ventura 3” – if Hollywood had made one – was all that was within reach.

Stiller plays two roles in this third “Museum” piece: Larry, the night watchman who knows the museum’s exhibits come to life after hours thanks to some ancient Egyptian magic, and a Neanderthal exhibit made in Larry’s image.

Larry has been promoted and looks bored and perhaps annoyed with the makeup that casts dark stripes on the splash of orange that they put on his face. Or maybe that’s Stiller.

But “La” grunts and gestures and mugs for the camera, John Belushi-style, as Stiller’s hairy/monobrowed doppelganger. La finds a laugh while Larry is basically reduced to high-priced straight man.

Not that anybody has a lot of comic wiggle room here. The Museum of Natural History’s exhibits are in danger of losing their after-dark/after-lives because of the failing powers of an Egyptian tablet. Larry and his friends slip off to London, where the parents (Kingsley included) of young Pharaoh Ahkmenrah (Rami Malek) may know how to save the day. Larry drags his aspiring DJ son (Skylar Gisondo) along, to no purpose.

The British Museum is guarded by Rebel “Pitch Perfect” Wilson, a funny lady who manages a couple of modest zingers in her few scenes.

But Stevens is the real find here, cut loose from his pale, lovesick turn on “Abbey” to sport long hair, armor and a dashing swagger as Lancelot, a knight in need of a quest. But even he never rises to hilarious.

Robin Williams, in his last film role, returns as Teddy Roosevelt, whose symptoms as the tablet’s magic wanes have him impersonating everyone from FDR and JFK to Reagan and W. – weakly.

It was Mickey Rooney’s final film, too, and he has but a single scene. At least Dick Van Dyke gets to show off his dancing moves, one more time. He, Rooney and Bill Cobb return as the original night watchmen, the ones who tried to rob the Museum of Natural History in “Night at the Museum.”

The Brits show the Americans up as only Stevens, Wilson and Ricky Gervais seem inclined to throw themselves into this paycheck picture.

Minor moments of slapstick may tickle the kids, but anybody older, especially those who remember what Williams was like in his prime and how funny Stiller was just two “Museum” movies ago, will wish this tomb had stayed sealed.