Three borderline cases wander about in a crumbling room.They are just this side of being the sort who shout at strangers on the street. Each keeps repeating his own long-ago set piece, only occasionally pulling one of the others into what remains a private act. At the end they wander off in their own directions again, untouched and unchanged.

And yet Harold Pinter's 1960 play, "The Caretaker," which is being done for the second time at the Olney Theater under James D. Waring's direction, is freshly fascinating.

Thomas Toner plays an elderly, belligerent bum, as he did at Olney 16 years ago; Stanley Anderson, a mild and orderly mental case who takes him in; and David Snell, the latter's younger brother, who alternates meanness and sociability like a flashing neon sign. They are intensely interesting portraits.

Why? Because, written and acted with pathos and wit, they succeed in an artisstic realism that is rare in modern theater. The social and psychological schools of drama are committed to the logic of internal cause and effect. What a character is on stage can always be documented by what society or individuals did to him. We're used to people who are the sum of their experience.

But these three characters are made out of tatters of miscellaneous modern urban experience, crazily sewn together to cover - we don't know what, if anything. They have snatched at human postures such as in dignation, dignity, kindness, imaginative reflection and toughness, and put them together in contradictory combinations that seem truthful.

The old bum haughtily rejects a new pair of two-toned leather shoes, after a litany about his inability to go on with his feet in rags. Is it because this would call his favorite bluff about wanting to walk to where his identity papers are supposed to be kept? No; he later replies nastily to the offer of a warm checked shirt by saying that he wanted a striped one. Just because the ability to reject does not fit into his life as a derelict, he is not willing to forego this common human pleasure.

The two brothers are able to offer him a variety of insults and treats, the ultimate one being the "job" of caretaker to their deserted dwelling. In the elder brother these impulses seem genuine, because he is not adept at playing, the way the younger, more socialized brother is. But the manner in which they have excerpted such traits, with no reasonable pattern, is also human.

That, it seems, is the dream of these characters who keep saying that they do not dream, who keep maintaining that they are on the verge of getting themselves together. Each believes he has managed to carry off, in front of the others, the presentation of a dignified and reasonable person. Such dreamers, presented on stage, seem real. CAPTION: Picture, PLAYING IN "THE CARETAKER" AT THE OLNEY ARE, FROM LEFT, STANLEY ANDERSON, THOMAS TONER AND DAVID SNELL