A witness for the NAACP charged today that a disproportionate number of black students are in classes for the learning disabled and mentally handicapped in the Prince George's County schools, and charged that such placement constitutes unjustified "legal segregation."

But on the third day of the county's school desegregation trial U.S. District Court Judge Frank A. Kaufman continued to criticize NAACP lawyers for failing to present detailed evidence on a case-by-case basis. He said he might call his own expert witnesses to furnish information about the NAACP allegations.

As lawyers for the civil rights organization neared the end of their presentation today, Kaufman asked witness Wylamerle Marshall if she had sampled individual cases of placement in Prince George's special education programs. Marshall, coordinator of programs for mentally handicapped as well as gifted students for the Dade County, Fla., schools, who testified as an expert witness for the NAACP, said she had not been asked to do so.

"Before experts testify, if they could do some sampling on actual facts, it would be better," Kaufman said.

"If you think you're putting on enough evidence , okay . . . but if I need to call your expert witnesses at your expense and get them to look at these documents I will," the judge said.

NAACP lawyers continued to present their statistical case alleging that the county schools harm black students by placing a disproportionate number of them in classes for handicapped children and that blacks also receive a disproportionate share of disciplinary action.

Marshall said Maryland Department of Education figures show that in 1980 blacks made up 60 percent of county students in classes for various categories, especially the learning disabled, while they made up only 49 percent of the overall school population. In the most restrictive category of special education classes--requiring entirely separate instruction apart from normal students--67 percent of the students were black.

Marshall said subjective judgments by school administrators and teachers are responsible for misplacement of some black students in special education classes.

"The effect would be harmful if you consider that the student is getting a curriculum progression that is different from what has been determined as necessary for the regular student," Marshall said.

School spokesman Brian J. Porter insisted that students are assigned to special education classes because of need, not bias. "We consider our special education programs to be among the most excellent in our system. We can draw no broad inferences from how the statistics fall," he said.