A. R. Gurney Jr. is very much Off Broadway's triple-threat man of the hour and of the season. After an auspicious debut at Playwrights Horizons, Mr. Gurney's ''The Dining Room'' has been running happily at the Astor Place Theater since last April. His ''The Middle Ages,'' originally showcased by the Ark Theater, is scheduled to begin rehearsals in mid-February. Meanwhile, ''What I Did Last Summer'' has been given its delayed opening by the Circle Repertory Company on Sheridan Square.
In this bittersweet memory play, Mr. Gurney revisits the upstate New York milieu which is his own native heath and favorite setting. ''What I Did Last Summer'' takes place at a summer colony on the Canadian shore of Lake Erie, a spot favored by neighboring Buffalonians. In the uncertain summer of 1945, Grace Higgins (Debra Mooney) is trying to maintain a vacation-as-usual atmosphere in the midst of wartime rationing and her husband's absence overseas on the Pacific front.
Grace's most troubling problem is coping with 14-year-old Charlie's unruly behavior and incipient independence. Rather than accept the customary lawn-mowing chores considered suitable for a lad of his comfortable, middle-class station, Charlie (Ben Siegler) goes to work for Anna Trumbull (Jacqueline Brookes), a reclusive, antisocial character known locally as ''the Pig Woman.'' A former art teacher who claims royal Indian ancestry, Anna disdains the well-to-do summer colonists who have contributed to her isolation. The impetus of ''What I Did Last Summer'' springs from the iconoclastic Anna's efforts to shock and scold and needle Charlie into realizing his creative potential.
As a genre comedy, ''What I Did Last Summer'' deals affectionately and humorously with the minutiae of a middle-class America, its domestic routines and crises. Charlie's sister Elsie (Christine Estabrook) has received her automobile operator's license but is too timid to drive. Superior and persnickety, she is deep in ''War and Peace'' as she marks summertime before becoming a freshman at Mount Holyoke College. Charlie's obsession with cars helps precipitate the crisis that climaxes this latest Gurney retrospective. Like other youngsters in his situation, Charlie doesn't become a man in the summer of '45. But he does grow taller.
As usual in a Gurney play, social comment avoids the portentous. The author's outlook and perspective emerge naturally in the attitudes of his characters, their intense concerns, even their absurdities and affectations.
The relationship between Anna and Charlie is touchingly realized by Miss Brookes, who resists any temptation to sentimentalize, and by Mr. Siegler, who eschews cuteness. Thanks to the skilled ensemble playing of a cast that includes Robert Joy as Charlie's patronizing Canadian friend and Ann McDonough as a delectable girl next door, the dialogue of ''What I Did Last Summer'' seems to play itself. And that is a tribute to the production, supervised by B. Rodney Marriott. (There is no directorial credit.)
The atmosphere of time and place is enhanced by John Lee Beatty's all-purpose , unpainted wooden setting, Jennifer von Mayrhauser's costumes, Craig Miller's lighting, and the fragments of pop recordings that help recapture an era.
''What I Did Last Summer'' is due to end its run on Feb. 20. This summer deserves to last much longer.