Agatha Christie’s full-length novels are significantly better than her short stories. Park Square Theatre’s trio of short works, “Rule of Thumb,” reveals that’s true of her plays, too.

It’s hard to know where to focus in the first two plays, where everyone seems to be a supporting character awaiting a lead who never shows up.

The best is saved for last. There’s a juicy star part in “The Wasp’s Nest”: the Belgian dandy who is Christie’s most famous detective, Hercule Poirot. Bob Davis has fun with Poirot’s accent (Christie indicated that he exaggerated it when it suited him) and confidently holds center stage while a puzzle — the sleuth hopes to prevent a murder — takes shape around him. The plot is simple, which makes it a good fit for a short play and for the story from which Christie adapted it.

On the other hand, it’s asking a lot of a short story or a half-hour play to concoct a whodunit while supplying believable motives and characterizations for several would-be killers. That’s the situation in the least successful play, “The Patient,” in which the denouement races by so quickly that I missed why the person who dun it, dun it. Director Austene Van, presumably aware of these shortcomings, smartly dolls up “The Patient” with stylized effects and acting.

Sara Richardson is amusing as a brittle socialite who appears to have learned everything she knows about being a human from Bette Davis movies, and Peter Christian Hansen drolly leans into his character’s oafishness. But the highlight is Audrey Park. Specifically, her retinas. She plays the paralyzed, mute victim of a murder attempt and it’s entertaining to watch Park, strapped to a hospital bed, deliver a witty and frightening performance with little more than eye movements.

Van also encourages the actors to go for big effects in “The Rats,” the claustrophobic opener. As a manipulative fop, Ryan Colbert appears to have dropped in straight from playing the Emcee in a production of “Cabaret,” and Richardson cleverly suggests her character’s hidden motivations.

As in any collection of Christie short stories, there are pleasures to be found in “Rule of Thumb” (extra credit to designer Sarah Brandner; her balanced set would pass muster with Poirot, who loved “the symmetry”). But it’s impossible for three mini-mysteries to be as satisfying as a richer one such as “The Mousetrap” or “Witness for the Prosecution.”

If you’re a Christie completist like me, you’ll enjoy recalling the stories on which these plays are based, or if you’re a newcomer, they offer at least a hint of the humor and invention to be found in her bibliography. But next time Park Square tackles a Christie mystery, I hope they opt for a full-length piece, one with a plot and characters we have time to get to know.