Once they have played the Beethoven Triple Concerto (as they are doing several times this week with the National Symphony), the brothers Piskunov and Yo Yo Ma have exhausted the basic repertoire for violin, cello, piano and orchestra. There aren't many triple concertos (just as there aren't many triple interviews) because it becomes tricky balancing the voices and themes for so many participants.

In an interview, unlike a concerto performance, cellist Ma trends to dominate the ensemble, mostly because he is more extroverted but partly because he has been living in this country since he was 7 (that would be 15 years). The Russian brothers Mark (19 years old: violin) and Alexander (23, piano) came to this country in 1973; Alex says they find it "more comfortable" than Russia, but he seems more comfortable at a keyboard than in an interview.

The three have played the Triple Concerto together once before, last summer in Aspen, Colo., and would like to - probably will - do it again but when? Next week, they go their separate concertising ways: Ma to California, Mark to Puerto Rico, Alex to the Juilliard School, where they all studied.

Mark plays in a string quartet of Juilliard whose cellist is Olga Rostropovich, daughter of the maestro. It's a small world. It's also a big world. Yo Yo ("My friends call me Yoki - like Yoki Bear") estimates that he logged 50,000 miles traveling to his 63 performance dates last year, and finds that one gets tired of hotel rooms and, "It can be a lonely life."

"I don't think I'd like to be realy famous," a theme stated by the cello with violin and piano harmonizing. "I want to be faithful to what I believe in. In classical music there are some people who are very sensational . . ." (two bars of discreet silence: no names).

" . . . We want to be diversified - compose, teach (counter-theme from violinist Mark: "I don't want to teach . . ."

"We could coach chamber music, play with orchestras (violin counter-theme: "Not in orchestras"). There are a lot of choices open to us - we want to try them and see what we like."

Mark became a violinist and Alexa a pianist not quite by chance but because their parents assigned each to his instrument when he was young. The parents had a very large (though not classical) record collection, and the boys lived in the sound of music from infancy.

"We got an impression of harmony and melodic lines from these records," according to Alex. "Later, when we started on classical music, we found it was not too different: it was the same language saying more complicated things."

Actually, all three play the piano, although only Alex will be it in public. "Mark plays the piano quite well," according to Alex, who should know. "He will sit down and play the most difficult part of something I am practicing; but only for a little while. He doesn't have time to practice."

All three also compose. Cellist Ma composes music for the piano. "As a cellist, I sometimes feel frustrated. Unless you are doing Kodaly's Opus 8 of the Bach Sonatas for unaccompanied cello, you always play with a piano or orchestra; you don't have complete control over the music, the way a pianist does."

Alex just sits there, smiling.