A three-day festival lavishly titled "Schubert, Schubert, and Schubert" opened last night in Georgetown University's Gaston Hall with some of the most gemu tlich music-making this city has heard in years. Its title could have been even more like the name of a law firm: Schubert, Eybler, Moscheles & Weber--without even mentioning Beethoven, who had a brief, charming trio movement after intermission.

The program included quite a bit of repertoire (by Eybler, Moscheles et al.) seldom heard in Washington. The underlying idea of the three programs (which continue through tomorrow evening) is to revive the "Schubertiads," the evenings of informal music-making that Schubert often enjoyed with his friends, performing not only his own music but that of contemporaries. It mixed serious music (which reached a historic pinnacle in 19th-century Vienna) with playfulness, which has been for centuries one of that city's prime export commodities.

At its best, the music blended both qualities. In Carl Maria von Weber's Violin Sonata in E-flat, Op. 30, No. 4, for example, a fairly serious Moderato opening movement leads into a rollicking polka before the music decides it has said enough. Schubert's piano Fantasia in E runs through a dazzling variety of moods, and the German Dances by Moscheles toss in some fetchingly lyrical violin solos amid their rustic dance rhythms.

Schubert provided most of the evening's seriousness, in his Sonata in A minor for violin and piano, in his delicious Notturno in E-flat, and in the exquisite song, An die Musik, sung by Gordon Hawkins in a voice of enormous, superbly controlled power.

Stars of the evening and the festival are the Viennese trio of Eduard Melkus, violin; Hubert Koller, cello, and Claus-Christian Schuster, piano--ably aided by violinist Fumiko Wellington in several numbers. Melkus is the best-known member of this group, through his many recordings, but this trio is not a violinist with accompaniment; it is an ensemble of equals, whose dialogue is not only skilled but deeply imbued with the styles of its native city.