The Wolf Trap Opera ended its summer season yesterday afternoon with its annual showcase of the singers who were in this summer's productions. Instead of the usual potpourri of individual arias, this summer the company came up with a better idea--doing acts from three different operas.

The 15 singers all had moments of display as they performed a part of "Rigoletto," Act II of "La Traviata" and the last 30 minutes or so of "La Bohe me."

It was a better day for baritones and sopranos than for tenors and mezzos.

Soprano Nicole Philibosian and baritone David Hamilton did some of the finest work of the afternoon as Mimi and Marcello in "Bohe me." Philibosian has a large, gleaming soprano that she used with tenderness. Hamilton has a dark, strongly focused baritone and sang with real dramatic conviction. His Marcello would stand up in any house.

Tenor Colenton Freeman had less luck with Rodolfo. Both his singing and the interpretation were too loosely focused. It is, of course, a difficult role, and perhaps Freeman was inexperienced in it.

That is, in fact, one of the problems with using full acts for the showcase. Certain singers will inevitably be less familiar with their roles than others.

As Musetta, Dale Wendel was bland. Peter Atherton sang a good Colline. And John Trout, as Schaunard, had so little to sing that one hesitates to judge.

In "Traviata," it was once again the soprano and the baritone who stood out. Lee Velta sounded especially distinguished as the elder Germont. He has a large, resonant baritone with considerable evenness of tone and an especially lovely top. And, even in this concert version, his stage presence was strong.

Katherine Henjum's Violetta was small voiced but fluent, and she, too, showed considerable presence.

Hal France, who is an associate conductor at the Houston Grand Opera, conducted the "Traviata" with real power.

Edward Randall's Alfredo was less distinctive. His voice seemed too small and his phrasing was flabby.

In the "Rigoletto" excerpts the pattern was the same. Soprano Susan Pierson was a lovely Gilda, with an impressive "Caro Nome." She has a sweet pure sound. And baritone Brad Liebl was strong and full voiced in the title role. Tenor Michael Brown was less assured as the Duke.

Mezzo Jennifer Larmore was a nice Maddalena, and Dale Ganz, who had been the splendid star in the production of Donizetti's "The Tutor's Dilemma," did not have all that much to do as Sparafucile.

There was also an aria from Massenet's "Werther" that soprano Meredith Wren Parsons performed ably, though not with great force.

Richard Woitach, who is the Wolf Trap Opera's music director, conducted all but "Traviata" and he did so with characteristic facility.