THE SPANIARDS were never the same once they had conquered Mexico. The terrifying power of what we rather belittling call "pre-Columbian civilisation" - the grotesque, giant basalt heads of the Olmic. . . . the pyramid cities fo the Maya . . , the gold vessels of the Mixtec . . , the memorials to the humans sacrificed by the Aztec - transformed mordern art.

The Mexican soil also transformed mordertn art.

The three great transformers, so to speak , are Jose Clementa Orozco, Diego Rivera and David Alfaro Siqueiros. The Hirshhorn Museum devotes an exhibit to these Big Three until April 30.

Now here, whether you like them if not, are the 20th-century Apostles of Social Meaning of Art, the Rejectors of Abstraction, the unintentional Fathers of (shudder) Social Realism and Heroic Monumentality.

But is show barely hints at any of this. It shows us some black-and-white photographs of the great fresoes, and the photos have the impact of a paper-weight plastercast of the Venus of Milo. Most of the show presents all but irrelevant homework "los tres grandes" did while they studied in Paris and made friends with Picasso, Derain, Braque, Gris and Modiglina.

In short, this exhibition does not tell us very much about is heroes. It tells us more about the people who puts this show on the road. The title, for instance, is interesting.

Twenty years ago, the art establishment would have named Diego Rivera first, considering Orozco a poor secong and Siqueros merely "interesting".(Ten years ago, in the heyday fo abstract expressionism, our art fashion leaders wuld have no more devoted an exhibition to these Mexician mural- ists than they would have worn fedora hats ans fur stoles to the opening)

The more than 35 paintings, drawings and prints by the three are, of course, an adjunct and sequel to the larger exhibition "Treasures of Mexico: From the Mexican National Museums" on display at the Smithsonian Museum of Natural History. The Hirshhon show is the follow-up, so to speak. It is to bring us up to date.

But what it tells us, at best is that here were three extraodinarily talented Mexican painters, who early in this century went to Paris, picked up the current art trends there - mainly Cubism - and were seemed by the terrifying spirits of Mexican-Indian antiquility and the Mexican revolution when returned.

But that is only part of the story. If may be that makes some of Rivera's stoic faces look like Aztec gods, that puts the harsh violence into Orozco's paintings and that embues even a Siquerios power of Miztec war god Huiziopochtil , who was offered hundred of virgins for his feats.

But this not for which we, more squeamish Morth Americans, invited Orozco to paint murals at Dartmouth and Pomona Colleges and at the New School for Social Research and other places.Nor is that the reason Diego Rivera was probably the most popular artist in North America in the late '20s and early '30s, when he did huge murals in the court of the Detroit Museum of Fime Arts and when the management destroyed the murals he had painted in a lobby of Rockefeller Center because he put Lenin into picture.

Siqueirios, too, worked in the United States. In New York in the mid-30s, he conducted workshops experimenting with outdoor mural paints, notably Duco, proxeln, proxeln and synthetic resins. In Los Angeles he painted several murals. They were tame compares to what he did at home. But in the end he was asked to leave, anyway.

Siqueiros was the most revolutionary of the three. Rivera, who tought his biography shoud be titled Rivera's "Wives and Times," rather thant "Life and Times," who liked unproletarian comforts and was constantly in and out of the Communist Party, dreamed of the painted ideal social justice for saintly Mexican peasants and workers, rather than agitprop.

Orozco, too, was not a dedicated Communist but a dedicated painter.Only Siquieros was constantly organizing and agitating, and in and out of jail. In the end he implicated in the plot to assinate Leon Trotsky, but nothing was proven.

The big three, in short,were not fighting the Communist revolotion. And their important rebillion was not against their vague concept of the capitalist system but against abstraction in art. It was not an abstract rejection of abstraction, as it were. Nor was it reactionary clinging to mawkish representationalism, as in some European schools of the time.

Motivated by compassion and by Mexican pride, they believed that the aloof intellectualism and nonrepresentational painting and sculpture could only lead to a dead end, that art that had no meaning for people, had no meaning period.

They went further: They felt that in our confused worlf "art for art's sake" was a decadent luxury, that art was a matter of edification - about the gods or God, past or future, hope or damnation and most of all about the mature of beauty.

The mesage the Mexicans chose was the dignity of labor, or call it "the heroism of the laborer." Siqueiros wrote innumerable manifestos about it, repudiating easel-art "and all such art which springs from ultra-intellectual circles, for it is essentiallly aristocratic.

"We hail the monumental expression of art because such art is public property."

The thought that easel-paintings, sold in art gallaries, had no buyers at the time, while monumental murals on public properties such as the Mexican Ministry of Education and school buildings in various parts of Mexico were generously commissioned by an understanding government crossed his mind only incidentally, of course.

In any event, after extensive study of mural painting, particularly the techniques applied in Renaissance Italy, los tres grandes wrought the great mural revival.

In Mexico, revival of monumental art had an instant effect on architecture. Enture building facades were covered with murals , some of them mosaic murals. The modern dilemma of ornament versus functional structure was solved. The entire building - a movie palace, government buildings, school houses, the library at the University of Mexico and others - because a colorful ornament. Some were painted nu Rivera himself. The most interesting mosaic work was done by Joan O'Gorman.

The Mexican murals were soon reflected on thousands of post office walls throughtout the United States, sponsored by the New Deal's Work PRojects Administration or WPA, which thus put thousands of starving artists to work. The most prominent American to take up the Diego Rivera genre was Thomas Hart Benton. He Americanized it with an air of Sunday school morally and a taste of corn. Hart also shares the belief that only an art that is meaningful to people is meaningful.

The belief,alas, was shared by Joseph Stalin Adolph Hitler adn Benito Musolini. Super-sized and super-heroic "social realism" a perversion of Riveras and Orozoco's mural style, became the hallmark fo totalitarian art.

But then, anything can be turned to kitsch.