JOHANNESBURG, JUNE 30 -- Ten black political leaders in a major treason trial in the farming town of Delmas, east of here, were released on bail today after nearly three years in custody.

They join six others who were granted bail last November, leaving only three of the 19 defendants in prison.

Judge Kees van Dijkhorst said he was prepared to free the 10 on $7,500 bail each because it was clear that the racial unrest that has shaken South Africa for nearly three years has died down and that there was a "material change" in the security situation.

He ordered them to report twice daily to their local police stations and prohibited them from attending political meetings.

The judge refused to grant bail to the three others, all key leaders of the United Democratic Front (UDF), the major black activist movement operating here, because he said he could not be sure they would appear for trial if they were released from prison.

While requesting a mistrial three months ago, the three said they regarded the judge as politically biased and did not believe they would receive a fair trial from him.

The three -- Popo Molefe, general secretary of the UDF, Patrick Lekota, its publicity secretary, and Moses Chikane, a provincial secretary -- were detained without charges under South Africa's stringent security laws in April 1985.

They and the other accused later

The judge said this compromised the assessor, a respected law professor, because the petition featured in testimony in the case.