Final design concepts that would transform part of the historic Federal Triangle into a major beauty spot with outdoor cafes, sculpture gardens and exhibits were disclosed yesterday to a General Services Administration Competition jury.

The plans, which represent the final three design concepts from among many that have been proposed for the area over the yeare, were drafted by several well-known architectural firms as a preview of their ideas for "opening up" the area around the historic Old Post Office Building 12th Street and Pennsylvania Avenue NW.

In a broader sense, the GSA design competition is just one small, but important, part of an overall plan for revitalization of Pennsylvania Avenue between the White House and the Capitol. Under the supervision of the Pennsylvania Avenue Development Corporation, noted architecultural consultants hope to turn avenue in to the grand boulevard envisioned by Pierre Charles L'Enfant in 1791.

Proposals for sprucing up the avenue include the renovation of the famous Willard Hotel at 14th and Pennsylvania; development of an office-shopping-hotel complex on the opposite corner where the National Press Building now is situated; relocation of a new Canadian government chancery across from the National Gallery; and a shopping mall in the former Lansburgh's department store.

The GSA's four-member jury must now decide which concept would best suit the varied needs of tourists, city residents and the 20,000 or so U.S. workers who use or visit that section of the city.

Although the three proposals varied in significant ways, they agreed in general that the triangular strip of imposing but drab federal buildings - bounded by 15th Street and Pennsylvania and Constitution avenues - should be drawn into the same kind of varied activities already enjoyed by the F Street shopping area to the north and the mall of the south.

In additional to proposals to make the area more esthetically pleasing to passer-by and government workers, the three design submissions are attempting to resolve the architectural dispartly of the Triangle's present buildings and to better organize and streamline the various transportation services now provided in the area.

One particular "problem point" in the entire Federal Triangle concept is the eventual sitting for the Old Post Office Building, constructed in 1899.

Earlier proposals for the Federal Triangle had been drafted under the assumption that the post office building would be torn down to make way for creation of what has been called the Great Circle to be formed by the colonnaded facades of adjacent building that would form its perimeter.

The Great Circle was only Partly completed, Known today as "Delano's Hemicycle," it forms a crescent-shaped plaza along 12th Street at one entrance to the new post office building.

For years, there was much discussion about completing the Hemicycle, but in 1975 the National Capital Planning Commission voted to halt forever planned demolition of the Old Post Office Building because of its historic significance to the avenue's grand design.

With preservation of the Old Post Office Building now essential, the finalists in the design competition yesterday discussed proposals for completing the ends of the Internal Revenue Service building adjacent to it while at the same time establishing a new setting for the older post office structure.

In an early morning presentation, Jerry Karn, vice president of the Harry Weese and Associates architecultural firm, said it is essential that "people be able to move through" the Federal Triangle area despite its formidable architectural scale.

His firm's plans would create a pedestrian zone area at the entrance to the Hemicycle and convert 12th Street into a three-lane northbound roadway with 10th Street converted to a southbound roadway in the triangle area. In addition, the firm would complete the ends of the IRS building and add more space to its inner court to make the setting more intimate.

Karn said the IRS arcade would house international restaurants and feature a water pool area that could be converted into a courtyard stage for ethic dancing and music.

The Hemicycle, Karn said, would be treated as "the gateway to the city" and lined with 10 water jets but no trees. The firm would also incorporate a Federal Walk concept into the design in an attempt to link the 11 agencies situated in the triangle area.

In a later presentation, the firms of Sert Jackson and Associates and Jerome W. Linsey and Associates detailed an architectural concept for the triangle that would focus on the old post office building.

Calling the present triangle design "uninviting," Lindsey said the firms proposed to extend the old post office structure at the first and second levels by building multilevel stairway entrances out to the street.

The firms also would remove or "square-off" a portion of the remaining IRS arcade, "erasing a broken piece of the unfinished circle and leaving in its place a complete Hemicycle across 12th Street."

Under a plan described by Lindsey, 12th Street would remain as a two-way street would be narrowed and 10th Street would be transformed into a bus plaza with more convenient and sheltered waiting facitlites than now exist.