Architects in three cities are busy this weekend putting the final touches on designs that will (a) alter our perception of one of Washington's leading landmarks; (b) challenge our ways of thinking about combining new buildings with old ones; or (c) cause a great commotion.

The correct answer to this multiple-choice question? All of the above are sure to happen.

The designs are for an addition to the Corcoran Gallery and the Corcoran School of Art, which jointly occupy the distinguished century-old building at 17th Street and New York Avenue NW, near the White House. When members of the Corcoran's selection committee made their choices last fall--narrowing an initial list of more than 200 architectural firms to three finalists--they guaranteed that the building would not be done in an old-fashioned way.

Chosen were Santiago Calatrava, a Spanish architect-engineer operating out of Zurich; Frank Gehry, a Canadian-born American artist-architect whose home base is Santa Monica, Calif.; and architect-theoretician Daniel Libeskind, a Polish-born U.S. citizen educated in Israel and New York who lives and works primarily in Berlin.

The trio will bring their competing ideas to the Corcoran this week. The winning proposal is expected to be announced later this month. Work on the new building and renovations to the old one, which together could cost about $40 million, will not begin until next year. Completion would be in late 2003 at the earliest.

In addition to their obvious cosmopolitanism, the three architects share a belief in artistic originality and a commitment to contemporary means of expression. Each has developed a highly individualistic design approach. It would be impossible to confuse a building designed by any one of the three with a work by any other architect on the globe.

Thus, their responses to the Corcoran challenge are sure to differ dramatically not only from one another's, but also from the more traditional ways Washington architects often have handled the contextual issue of fitting the new alongside the old. This is good--Washington seems to cultivate a certain smugness in such matters. Fresh ideas are needed and welcome.

Yet there are risks. The Corcoran, designed by Ernest Flagg and completed in 1897, with a largely hidden 1928 addition by Charles Platt, is a very fine building.

It occupies an important corner with grace and dignity. Its long, elegant 17th Street facade is unlike any other in the city. Its steep, elegant roof is in itself a landmark. Inside, the main stairwell and rotunda make a splendid pair, and the atrium with its double-stacked colonnade is at once noble and intimate. The spacious, naturally lit second-floor galleries of the Flagg building are among the best in the world for displaying paintings.

You don't tinker lightly with such a treasure. At the same time, respect for the architecture doesn't necessarily mean that, when adding to it, you have to copy it, which basically is the approach the notable Washington firm of Hartman-Cox took when designing a proposed addition for the Corcoran in 1987.

The Hartman-Cox design didn't get built for a variety of reasons. One was administrative confusion. The Corcoran, a private cultural institution struggling to find its way among Washington's federally supported goliaths, was suffering one crisis after another. The infamous 1989 cancellation of a controversial exhibition of photographs by Robert Mapplethorpe, and the consequent resignation of the Corcoran's director six months later, was symptomatic of the institution's woes.

A second reason was the speculative nature of the proposal--it was intended to be a money-generating rental office building attached to the Corcoran in the manner of the Cesar Pelli-designed skyscraper next to New York's Museum of Modern Art. When the bottom dropped out of the Washington real estate market in the early '90s, the project was put on indefinite hold.

The third, decisive reason was the appointment of David Levy as director in 1991. Levy, the longtime director of the Parsons School of Design in New York, hated the idea of placing an office building on the adjacent empty lot facing New York Avenue. The school desperately needed additional space, and Levy believed strongly--and correctly--that the best possible solution was to expand right next door.

Furthermore, though he did not say so publicly, Levy let it be known that he disliked the Hartman-Cox architectural approach. He favored a bolder direction, and, biding his time to build trustee support and a sounder financial base, he got what he wanted.

The Hartman-Cox design was characteristically circumspect. A rather large building, it took all of its stylistic cues from Flagg's original. The cornices lined up, the windows were given the same kind of surrounds, and ornamental themes were duplicated, as if the building had been designed by Flagg himself or by a close contemporary.

In retrospect, however, it seems doubly fortunate that this building was not built. The office use was terribly wrong, and the architecture, though deferential, today seems swollen and stodgy.

By 1987 Hartman-Cox (with George Hartman and Warren Cox as founding partners) had fully abandoned its own origins in modern architecture in favor of polite, historical eclecticism. The city of Washington has benefited enormously from the firm's skillful sensitivity to the urban fabric--Hartman-Cox designed more than half a dozen major buildings here in respectful historical styles during the '80s. But the proposed Corcoran addition now seems over the top--superficially attuned to Flagg's original but reflecting little of Flagg's originality. (By contrast, Hartman-Cox's freer, early '80s addition to the Folger Shakespeare Library on Capitol Hill, combining stone with steel, respectfulness with wit, seems better and better.)

In any case, it is certain that none of the forthcoming designs will remotely resemble the 1987 effort. Calatrava, Gehry and Libeskind share a radical approach to contextual issues. Gehry experimented with ironic, postmodernist quotation in the early '80s, but it was a very brief fling. Otherwise, none of the three has shown the slightest interest in historical styles.

To the contrary, the styles of all three are strongly sculptural and abstract, though in idiosyncratic ways. Forceful contrast, rather than placid complementarity, is a leitmotif of all the work.

To point this out is not, however, to say that the three architects ignore context altogether. Although Gehry and Calatrava are best known for their dramatic, free-standing works, each has dealt with complex urban situations in his architecture, and each has been commissioned specifically to add to important historic buildings. As for Libeskind--whose career as a practicing architect has been brief--his only completed buildings so far are additions to older, existing structures.

It was Gehry's 1992 University of Toledo Center for Visual Arts, in fact, that taught me once and for all that an architect can be daring, imaginative and incredibly freewheeling and yet be responsibly contextual at the same time. Having seen photographs of it before visiting Toledo, I arrived prepared to dislike the building--it looked dark, foreboding and rude. Yet it was love at first sight.

An art school addition to the Toledo Museum of Art--a fine early 20th-century neoclassical building--the center is thoroughly unlike its parent structure: round rather than straight, sheathed in metal rather than stone, quirky rather than predictable in its rhythms. The list of contrasts could go on and on, but the point is that by carefully controlling just a few things--primarily scale and proportion--the architect made a building that is delightful on its own and complementary to its neighbor.

This is not a plea for a pro-Gehry vote by the selection committee. I am simply emphasizing that these are just the sorts of qualities one would hope to see in the Corcoran addition and its relationship to the original building.

Heaven knows, there are plenty of obstacles, both practical and aesthetic, on the way to a successful, let alone triumphant, outcome. There are certain problems with the Corcoran's brief to the architects. Most worrisome is the possibility it holds out of using every possible square inch and then some. This contributed to the somewhat bloated quality of the 1987 design and will put similar pressures on the new architect.

Then there is the prospect of closing off all or most of the natural light in those upstairs galleries. The idea, conceived by Washington architect Arthur Cotton Moore in a pro bono contribution to the museum, may be brilliant--it foresees converting most of the space between the roof of the old building and the glass ceilings of the galleries to classrooms and other facilities for the school. But such a conversion should be approached very, very carefully, for the quality of those rooms should not be compromised.

And there are, let's say, maybe a hundred other serious challenges--figuring out the proper distance between the existing roof line and the new building, connecting two complicated buildings in myriad ways, linking new public spaces with the old ones without detriment to the latter, establishing just the right identity for the new building on New York Avenue and so on. A certain balance is required. The addition should have its own identity, yes, but it ought not upstage the original.

In choosing these three architects, the Corcoran set an adventurous course. The selection and the discussions that follow should be exciting. My advice to all involved--the general public, the Washington architectural establishment and the agencies that will have to review any final design--is to keep an open mind, don't rush to judgment, hold on to your seat and enjoy the ride. The result could turn out to be exhilarating.

CAPTION: Hartman-Cox's 1987 plan paid homage to the existing structure at left. The new addition proposal is expected to be chosen later this month.

CAPTION: Corcoran finalists include Frank Gehry, who designed the unbuilt Disney Concert Hall in Los Angeles, above; Santiago Calatrava, whose commissions include Toronto's BCE Center promenade, left; and Daniel Libeskind, designer of the Imperial War Museum in England.