THE fifth concert of the NationalSymphony’s season, scheduled for tonight,May 6, and Sunday morning, May 8, is anall-Beethoven program featuring the rarelypresented triple concerto for violin, celloand piano, Opus 56.Janna Hymes-Bianchi returns to thepodium, leading soloists Jacques Sagot onthe piano, José Castillo on the violin, andÁlvaro González on the cello.Beethoven started work on this concertoin 1803, and it was premiered in 1808. Ittherefore comes from the same period asthe “Eroica” symphony, the “Waldstein”and “Appassionata” piano sonatas, and thefirst version of his only opera, “Fidelio.”The work was not well received, and wasnot performed again during the composer’slifetime.The interplay of multiple soloists andan orchestra seems to have heldBeethoven’s interest – the solo group in thetriple concerto is the piano trio, with violinand cello.The composer from Bonn, Germany,had already completed several piano trios,and had successfully balanced the differenttimbres of the three solo instruments.Then, he set himself the task of combiningthese timbres in a dialogue with a fullorchestra.Opus 56 requires three superior soloistswilling to work with each other and blendwith a full orchestra. It’s generally rare tofind this work programmed, as manyorchestras do not have the funds to compensatethree soloists, nor are they able toprovide the rehearsal time necessary. Oneassumes that the three Costa Ricansoloists, longtime associates here, have hadthe opportunity to prepare themselves overthe past few months.The cello dominates, and is generallyconsidered to be the most difficult of thesolo parts; Beethoven was familiar with thesuperior abilities of cellist Nikolaus Kraft(for whom Franz Joseph Haydn had alsowritten his cello concertos).Some musicians say the piano part isthe easiest, since the premiere was playedby Beethoven’s most important patron andregular piano student, Archduke Rudolphof Austria; however, most historians nowbelieve the composer had not yet met theyoung man when he began work on Opus56 in 1803.The other major composition on theprogram, the Fifth Symphony, tops the listof favorites on nearly every poll.Beethoven struggled over the piece foralmost a decade, and the three quick Gsand the long E-flat, the opening of theFifth, could be regarded as the most memorablemusical phrase of all time. This simplemotif unifies the entire work. Suchbold harmonic and structural explorationslaunched the Romantic Era of music twocenturies ago.Remaining tickets for both concerts areon sale at the National Theater box office,and are priced from ¢1,000-6,000 ($2.10-12.60). For information, call 221-5341.