Three cheers are too few for Katharine Hepburn. In "The Corn is Green" on CBS tonight, she plays a character who epitomizes what Hepburn herself has come to represent: lofty, impertinent resilience. Whether Hepburn really is playing Miss Moffat, the stubborn English tutor, or just playing Hepburn is immaterial to the tremendous amount of pleasure at hand.

Hepburn is Hepburn is Hepburn. "The Corn is Green" is Hepburn and she and it are magnificent to watch.

George Cukor, a longtime Hepburn handler, directed this new version of the Emlyn Williams story,to be seen at 9 on Channel 9, and it was shot on location in the green-green country-side of North Wales. The work is a sentimental anthem to whatever heroes we have been able to find in our lives and travels and to whatever challenges they helped us to meet; many of us feel we have been lucky enough to have known and been bullied by a Miss Moffat of our own. Perhaps one even boxed our ears somewhere along the way.

Hepburn's authority and magnetism in this rold -- once played on film by Bette Davis -- makes Miss Moffat invigoratingly tough and regal, but there is more to this performance than simple, splendid performing. The mere fact that Hepburn keeps working, that she comes to television with no air of condescension, but rather her usual determination, makes her all the more admirable a figure to behold.

Miss Moffat arrives at her newly acquired estate early in the film and sets up a school for the local children because it is the right thing to do. One of them, a miner's son named Morgan Evans -- played with commanding and convincing rough edges by newcomer Ian Saynor -- astonishes Miss Moffat with his eloquence and intelligence, and she sets out to free him of his working class shackles and send him off to Oxford.

Her demands on him are patently unreasonable yet she puts them forth with intimidating conviction.The boy suffers ridicule from his fellows and is momentarily waylaid by a buxom girl's seduction (she sings "My Sweetheart's the Man in the Moon" to him on Miss Moffat's staircase) but the story has the great and gratifying decency to end just the way one hopes it will. This contagious sense of triumph takes one back to the proudest moment of one's own childhood -- whenever it was, however distant it may seem, whatever a father or mother said that made one dizzy with victory.

It wasn't necessary to transpose the time or setting of this story to make it recognizable and involving, and the filmmakers deserve credit for resisting that temptation. Cukor has a few awkward transitions in the first hour of the drama but the exhilaration of the crucial sequences make s up for such lapses.

Perhaps the most affecting of these is a scene in which the amazed Miss Moffat reads aloud to herself from the boy's unlikely lyrical prose. While he, outside her house, ecstatically rings a school bell hung from a tree and jumps for joy into the air. He is elated at the prospect of the new world that has been opened up to him by the flinty old biddy with the cracked and trembling voice and the wherewithal of the Spanish Armada.

Hepburn's conquest of the lad takes a while; her conquest of the audience is instant. Yet she doesn't just rely on the decades of good will that she has earned on the screen. Her performance radiates both bravado and subtlety. She strides into a barn early in the film with a hearty "I like the smell of cow dung, con't you, Mr. Jones?" To bamboozle the bullheaded squire of the vilage into patronage of her pupils, she feigns a daffy daintiness, dons a cripplingly proper hat ("But it's so fluffy," she complains) and throws herself at his male chauvinism with extravagant cries of how "week, weak, weak," women are.In a pig's eye.

Brushing her hair back from her forehead in midspeech or riding to town in a carriage pulled by a plump white horse, Hepburn is majestic allright, but she is no slovenly monarch. She is still strving and exerting herself, and appearing as formidable as the Great Wall of China. When four unruly rowdies are warned, "Wait until you see Miss Moffat -- she will give you what-for," we know this is a promise that will easily be kept. She could give the whole world the what-for it deserv es.

Hepburn is an iron angel and the Corn Is Green" displays her in her glory. While you are watching it, it seems a tonic for everything that ever went wrong. A few more Miss Moffats, you think to yourself, and wouldn't life be better all around? For even one Hepburn, however, we can be thankful forever. CAPTION:

Picture, Katharine Hepburn in "The Corn Is Green," by Associated Press