It would not be an exaggeration to say that the three Dutch artists represented on this program are probably the best players of their respective instruments in the world. Gustav Leonhardt was not only a pioneer in the field of early-music performance practices, but one of the first performers to work with period instruments. He is arguably the most sensitive harpsichordist playing today, and his performances in the States are few and far between. Given his age and health this rare appearance could be his last in the area. Although Frans Brueggen has been trying to establish himself as an early-music conductor, with this concert he returns to the instrument that made him famous in early-music circles--the Baroque recorder. Cellist Anner Bylsma gets the sweetest and purest tone on a Baroque cello that one could ever hope to hear, and his artistry is always a pure delight. The three will perform both separately and together in a program emphasizing the early sonata in 17th-century Italy. Tuesday, 7:30 PM, Pick-Staiger Concert Hall, Northwestern University, 1977 Sheridan, Evanston; 491-5441.