'Alone Together': humor flows when three grown men come home; Alone Together Comedy by Lawrence Roman. Directed by Arnold Mittelman. Starring Janis Paige, Kevin McCarthy.

Janis Paige and Kevin McCarthy are bestowing their histrionic wiles and attractive personalities on an old-fashioned formula comedy about family life in California. Although set in the Golden State, ''Alone Together'' might take place anywhere in America where the Reagan tax cuts count.

Author Lawrence Roman imagines what happens when the grown sons whom George and Helene Butler (Mr. McCarthy and Miss Paige) think they have sent into the world decide, for various reasons, to return to the nest. The disenchanted Helene refers to the boys as ''recidivists.''

Chief among the homecomers is 30-year-old Michael (Don Howard), a subgenius and gourmet cook who speaks three languages, plays chess blindfold but has fled MIT, having failed to solve an insoluble math problem.In most respects, Michael behaves like an over-age adolescent; Mr. Howard gives him a very funny performance.

Mr. Roman gets all possible mileage out of the Butlers' handling of the sudden population explosion in their midst. As the conciliatory George, Mr. McCarthy personifies patience and perseverance until events drive even this paterfamilias to desperate measures. Miss Paige, looking terrific and playing with marvelous insouciance, completes the picture as a devoted wife and mother who has been hoping that freedom from domestic chores would liberate her to return to a painting vocation.

The multiplying menage is completed by Kevin O'Rourke as Elliott, an obnoxiously loud-mouthed womanizer; Dennis Drake as Keith, the youngest, whose motive for returning home is sheer flimsy; and Alexandra Gersten, as a charmingly daffy girlfriend who is big on doomsday and (fortunately for Helene's peace of mind) ''into celibacy.'' A dropout from another era, Janie is learning kung fu from Michael while teaching him what flowers can be eaten.

If ''Alone Together'' sounds like a one-joke comedy in triplicate, the description is not so far amiss. Mr. Roman is a fast man with a funny line, but it takes all the efforts of this fine cast to sustain interest as the play careens from crisis to crisis under Arnold Mittelman's energetic direction.