If all three acts of "Madame Butterfly" had gone as well as the last act on Saturday night at the Kennedy Center, it would have been a far more exciting and moving performance.

For a variety of reasons, all the crucial elements in the opera coalesced in the third act, wringing out the intensity and anguish of the tragic finale. John Mauceri, who conducted with the greatest sensitivity throughout the evening, timed every phrase for its ultimate effect. The lighting, which had been ideal in every scene, took on particular poetic impact as Butterfly's last day broke.

But the chief factor that gave the final scene its crushing impact was the acting and singing of Dennis Bailey as Lt. Pinkerton. From his entrance in the first act, Bailey proved, with impressive versatility, that he fits in the large reaches of the Opera House as easily as in the intimate spaces of the Terrace Theater. His voice opened up handsomely in the exchanges with Sharpless and with thrilling color and drama in the love duet. It was in the final act, however, that his strengths as a singing actor filled the stage with a sense of urgency that had been strangely missing in his absence.

The popular opera is one that demands, above everything else, a Butterfly who can shade and shape every phrase while fitting each gesture and each move into an elaborate mosaic. Patricia Wells, singing the title role in the Washington Opera's new production, is a Butterfly still in the chrysalis. From a vocal standpoint, she sings the part well, though she drives her voice dangerously hard in her final farewell to her son. Her offstage high B was a lovely pianissimo, but she does not give the role nearly enough light and shadow in the face of its endless opportunities. Neither her entrance nor her "un bel di" drew notable applause. More nuance is needed at every point.

At her best when engaging in the dialogues that fill the opera, Wells was fortunate in the skill of her colleagues. She had an excellent Suzuki in Judith Christin, who was another of the decisive forces not only in the third act but throughout the long, taxing second act. Her voice is richly dark and she handles it with finesse.

Richard Stilwell's Sharpless was acted with great skill, especially in the second-act dialogue with Butterfly and throughout the final act. He contributed some lovely quiet singing, but in large passages his voice tended to disappear.

One of the finest moments in "Butterfly," the scene for Prince Yamadori, has rarely been as effectively staged as in Frank Rizzo's handling. Much of its success is due to Joseph Galiano's aristocratic bearing and flair for movement and gesture. That Goro, the marriage broker, is a sleazy character is no news. But Andre Lortie can be congratulated for making the part seem unusually obnoxious. There are, however, better ways to play the part.

Noel Tyl was a properly menacing Bonze and Jeffrey Wells satisfactory as the Imperial Commissioner.

Another reason the last act was so deeply moving was the revelation of the role of Kate Pinkerton, the innocent cause of Butterfly's final grief. The minature role was beautifully handled by Gail Mitchell.

Rizzo's staging varied. The first act was cluttered with too much fussy business and too many new props that failed to achieve what can be dome more simply. The second act went better with a lovely flower scene that was vividly recalled in a stroke of genius in the last seconds of the opera. The third act was triumph, but with strange aberrations. Why have Butterfly and Suzuki wrestling at that door when a half dozen other characters simply walked in and out of the room by using the adjacent steps? It looked silly.

Patricia Collins' lighting deserves highest marks, as do Ming Cho Lee's setting , Zack Brown's costumes, and Charles Elsen's makeup and wigs. John Mauceri, with the utmost in sensitive conducting, drew beautiful playing from the orchestra and expert singing from the chorus.