Sunday night's Big Event on NBC (Channel 4 at 8) follows three rules of larger-than-life drama: Sophia Loren, Sophia Loren and Sophia Loren.

Entitled "Sophia Loren Her Own Story," with the same unpunctuated grandeur of the best-selling (auto) biography written (In Her Own Words) by A. E. Hotchner, this gloriously catalogued galivant upwards from poverty to cinematic omniprescence is going to set the Nielsen ratings on fire.

The reason, literally, is Sophia, of whom any right-thinking audience cannot possibly get enough. Her beauty and pride are in nearly every scene, her glamour and talent in every gesture of expression, however small. She is not only the star, but the solar system too. Several times she appears on screen three times simultaneously: as her mother, whom she portrays; as a 16-year-old, a big-eyed, shoulder-tossing impersonation by a knockout young American actress named Ritza Brown; and of course, as her self, because any fool can tell that she isn't her own mother.

In addition to herself, Loren brings her own best-selling story, and as drama it's better than most of her 70 motion pictures.

She was born Sofia Scicolone of Pozzuoli, Italy, on Sept. 20, 1934, the illegitimate daughter of Romilda Villani. Villani had won a Greta Garbo look-alike contest and aspired to a theatrical career, but it was not to be. That was reserved for Sophia, along with her tempestuous life with Carlo Ponti, who discovered her, and Cary Grant, who fell for her while they were making "The Pride and the Passion" in 1956. It is a plot nobody would believe, had everybody not followed it in the newspapers all along.

The first hour of "Sophia Loren Her Own Story" is scary. That is, one is afraid the program isn't going to be any good. Loren, playing her mother, is proud but reserved. Armand Assante, as Riccardo Scicolone, her father, is a two-timing scoundrel, but reserved.Scicolone, Loren confirmed on NBC's "Today" show yesterday morning, did in fact charge her mother a million lire to permit her sister (also illegitimate) use of the Scicolone name. Loren's reaction yesterday was, "He needed money . . . I mean, I can't talk badly about my father."

Perhaps the early drama was reined in out of respect for Loren's mother, who certainly seems to have been unlucky in love. But when Sophia has grown to 16, and Ritza Brown enters the picture, everything comes to life.

When Loren, still playing Romilda Villani, strides along next to Brown, playing Sophia, the effect is boggling -- particularly in the parallel nonpareil bodices of mother and daughter. Sophia meets Carlo Ponti, played by Rip Torn wearing a hat. Ponti falls in love with her, his marriage notwithstanding, but wants to get her a nose job. Fool! Sophia's nose is saved in the nick of time by Vittorio de Sica.

Meanwhile, her professional name is changed and Sophia starts playing Sophia in time to do the Cary Grant affair scene in person. John Gavin is the Grant stand-in. "There are three stages of Love," Gavin says, sounding like Frank Gorshin. "Attraction, infatuation and love.I'm at three." Sophia says, "I'm at 2 1/2."

She marries Ponti by proxy in Mexico, not knowing that the Vatican has disallowed his divorce. Poor Cary learns the bad news on the radio, and confronts her with his broken heart: "Hearing something that's going to turn your whole life inside out on the 8 o'clock news between the weather and the sports -- it's grotesque!"

Meanwhile her real father, that bounder Scicolone, is still causing trouble. Sophia has had several miscarriages, but carries the burdens of fame and fate well nevertheless. Ponti finally solve the marriage problem by becoming a French citizen, and he and Sophia are married, and the story stops in a blaze of glorious television color with the birth of Carlo Jr.

The first hour is slow; the theme music is vapid and execrable, and the language problem has been totally finessed here. There is no Italian spoken in these three hours, and after Shogun one hungers for mellifluous foreign tongues as the accompaniment to international settings. Loren herself has an accent, and some of the cast do too, and others do not, and the M.C. of the beauty contest in Rome sounds like he's in Dayton, Ohio.

But Sophia Loren, as both self and mother, is just plain stunning. It's her story, and she's perfect for the part.