FBI Director William H. Webster said yesterday that three or four of the Atlanta murders of young blacks have been "substanitally solved."

These cases are unrelated and are not the work of the mass killer or killers whom the FBI thinks committed 12 to 16 of the 23 murders that have outraged and frightened Atlanta.

"We're reasonably sure of the reasons and perpetrators," Webster said of the three or four cases.

Webster said, however, that he does not expect early indictments in these cases and said that the search for the mass killer or killers has not yet turned up evidence that leads him to expect a quick solution.

Reliable sources said the killers would not be indicted soon in the cases that are nearly solved because Georgia prosecutors don't want to start with these. The prosecutors want to begin with the mass killings, not these cases that are completely separate from one another and from the mass killings and involve circumstances and suspects more commonly found on police blotters, the sources said.

However, Fulton County District Attorney Lewis Slaton denied that there is any intention to wait for any particular case. "If any case breaks and we've got the evidence, we'll proceed. We will not hold up a case to wait for another case," he said in an interview with Washington Post reporter Art Harris.

Slaton said yesterday he is not aware of "sufficient evidence to indict anyone at this time." He said he is in daily contact with FBI investigators.

Webster said it is not clear from the evidence whether a single person or more than one killer operating in concert murdered the 12 to 16 youths. The latest victim to be found, Larry Rogers, 21, is one of these related cases, Webster told reporters.

The FBI director added that the discovery of Rogers' body Thursday in an abandoned apartment building proved investigators with their first crime scene. The other victims apparently have been murdered and then transported to the sites where their bodies have been found.

A number of "heartbreaking leads" that have "seemed so certain to take us there" and then have grown cold have frustrated the 30 FBI investigators and roughly 35 Georgia investigators working on the cases, Webster said.

Webster said they are "waiting for a break."

"We're getting to a point where we should have that break coming," he added.

Asked whether the Atlanta killings are motivated by hatred of blacks, Webster replied that the fact that all the victims are black "could be consistent with preference as well as with prejudice." He drew a distinction between the Atlanta killings and the racially motivated murders of blacks in Buffalo, N.Y.

Some of the Atlanta victims have been found wearing only undershorts. Medical examiners have been unable to find conclusive evidence of sexual abuse, but have not ruled out a sexual motive in the linked slayings.

Webster also remarked that the pace of the killings in Atlanta is accelerating. "There was a time cycle and that cycle got shorter and then got shorter again," he said.

In Atlanta a 15-year-old black youth with a history of running away was listed as a missing person yesterday, but was not added to the list of 23 who have been murdered and two young blacks who have disappeard over the past 20 months. Dexter Lee Jackson was last seen April 1 when he left his home to go to school.

Meanwhile, a spokesman for the Committee to Stop the Children's Murders, a group formed by the mothers of some of the victims, said plans are being made to hold a rally near the Lincoln Memorial on May 25 that the group hopes will attract 100,000 people.

On March 30 attempt to assassinate President Reagan, Webster said that the great bulk of evidence is consistent with the theory that has been widely printed: that the suspect, John W. Hinckley Jr., acted alone and was motivated by his infatuation with movie actress Jodie Foster whom he had never met.

"Nothing we have found reflects any hostility toward President Reagan or his family," Webster said.

He added that the efforts to trace Hinckley's movements over the last two or three years are making rapid progress.