Last night's concert by the National Symphony was an unusual triple header, offering a puzzling work by the contemporary American composer Jacob Druckman, violinist Elmar Oliveira in his debut with the orchestra and a finely wrought performance of Elgar's "Enigma Variations."

Least satisfying was the Druckman piece title "Mirage," which was one of the bicentennial orchestral commissions sponsored by the National Endowment for the Arts. It is an elusive work marked by false starts and purposeful strides to nowhere. Full of strong contrasts in mood and sound it makes use of an offstage orchestra which both echoes and contradicts the main orchestra onstage. Despite a well-controlled performance by guest conductor Christian Badea, the totaal effect remained tentative, just the way, of course, a mirage is.

Violinist Oliveira, who became in 1978 the first non-Russian to win the gold medal in the Tchaikovsky competitions, chose to make his debut with the lush, if less familiar, sounds of Glazunov's Violin Concerto. T Though not a showy virtuosic work, it demans the kind of luxuriant, deep tone and liquid style that Oliveira commands.

Badea's carefully balanced and refined direction turned the concluding Elgar work into the evening's high point. The orchestra produced some of its most sensitive playing to date. The strings were delicate and precise, the brass warm and full, the wood-winds graceful, and there were some lovely solos all around. Climaxes, when they came, meant something because they were beautifully controlled and clear.