Three young black musicians yesterday won cash prizes and offers to appear with the National Symphony Orchestra next season. Roger Stevens, board chairman of the Kennedy Center, announced the names of the winners yesterday at the final session of the National Black Music Colloquium and Competition.

A $2,000 prize was won by pianist Alison Deane of New York City, who has already made her formal New York debut in a Carnegie Hall recital. Another $2,000 was divided between bassist John Clayton of Los Angeles, who has played with the Count Basie Orchestra as well as given clasical recitals, and vocalist Marcus Thompson, who is a professor of music at Massachusetts Institute of Technology and has performed with the National Symphony and the Boston Pops, among other orchestras.

Besides her appearance with the NSO, Deane will be featured in an appearance with the Buffalo Symphony Orchestra. Clayton and Thompson will be included in this spring's chamber music series of the Piccolo Spoleto, U.S.A., in Charleston, S.C., and Yehudi Menuhin will provide the round-trip air fare for Thompson to complete in the Carol Flesch International Competitions for violin and viola in London. All three winners will also play with the Niagara Falls Symphony Orchestra.

Following the announcement of winners, the colloquium concluded with a panel, chaired by Archie Buffkins, president of the National Committee on Cultural Diversity in the Performing Arts at the Kennedy Center, discussing the establishment of a national policy agenda for blacks in the arts. "It is very important for black people," Buffkins said, "to make sure that projects do not terminate after the program is over."

Among the possible ways of continuing the impact of the colloquium, Raymond Jackson of Howard University suggested an apprenticeship system under which young instrumentalists would be associated for a year or two with a major symphony orchestra or a major soloist. Hildred Roach of the University of the District of Columbia suggested that the colloquium should become an annual event, and should include recitals by major artist such as Andre Watts and Natalie Hinderas.

Violinist Sanford Allen, who left the New York Philharmonic for a solo career, talked about the "isolation" felt by black players in symphony orchestras. "When we get together," he said, "we go through a cycle of horror stories . . . The only strength we have if we have any at all, lies in keeping as close contact as we can."

He praised the Kennedy Centers sponsorship of the colloquium as "one of the few times that a power center has decided not to be a reactionary as power centers usually get."