The Justice Department has gone outside the ranks of its career attorneys again to find an experienced trial lawyer to lead the prosecutions of three former high-ranking FBI officials accused of approving break-ins in search of fugitives from the Weather Underground.

John W. Nields Jr., the chief lawyer in the recent House investigation of South Korean influence buying, was named to head the trial team in the cases against former acting FBI direcotr L. Patrick Gray and, separately, against two of Gray's top aides, W. Mark Felt and Edward S. Miller.

Nields takes the place of Barnet D. Skolnik, a former assistant U.S. attorney in Baltimore who successfully prosecuted Vice President Spiro T. Agnew and Maryland Gov. Marvin Mandel. Skolnik entered private practice last month.

Philip B. Herymann, head of the department's criminal division, said the selection of Nields was not a sign that the department has a shortage of capable criminal trial lawyers.

"We need in these cases a unique combination of someone thoroughly familiar with national security procedures as well as someone experienced and talented in criminal trials and legal arguments." Heymann said. "John Nields has all these qualifications."

The case against the three fromer FBI oficials has been plagued by delays arising from defense claims for intelligence secrets. The department recently agreed to try Gray separately and is expected to try to go ahead first with the case against Felt and Miller.

All three are accused of violating the civil rights of relatives and friends of members of the radical Weather Underground by authorizing break-ins of their homes by FBI agents in the early 1970s.

Defense attorneys have claimed that the break-ins were legal because the fugitives had ties to foreign governments. They have been seeking national security docurments to prove that point.

Nields, 36, was an assitant U.S. attorney in the Southern District of New York for five years, and then senior law clerk to Supreme Court Justice Byron R. White.

For the past two years he was chief counsel to the House Committee on Standards of Official Conduct during its investigation of the Korean bribery scandal. In that inquiry, Nields gined much experience in handling national security documents. CAPTION: Picture, BARNET D. SKOLNIK . . . leaving for private practice