The Carter administration has assembled a list of three candidates to take over the Justice Department's antitrust division, sources said yesterday.

The three under consideration are:

Donald L. Flexner, the division's deputy director, who is now running the section while the assistant attorney general for antitrust matters, John H. Shenefield, is serving as acting associate attorney general.

Kenneth R. Reed, the chief counsel of the antitrust division of the Arizona attorney general's office.

Sanford M. Litvack, an attorney for the last 18 years with the New York firm, Donovan, Leisure, Newton & Irvine.

Although all three candidates refused to comment on reports that they were under consideration, authoritative sources said that a decision on the appointment is imminent.

Each of the candidates has met with Attorney General Benjamin R. Civiletti and all three appear to have bases of support.

Flexner has the support of Shenefield and a number of career Justice Department officials, although some Congressional critics of Shenefield's tenure in the job believe Flexner is too conservative.

Reed, who has been praised by Rep. Peter W. Rodino Jr. (D-N.J.), chairman of the House Judiciary Committee, Sen. Dennis DeConcini (D-Ariz.) and Rep. Morris K. Udall (D-Ariz.), also has the strong support of the National Association of Attorneys General.

Litvack, who is not particularly well-known among antitrust observers in Washington, is considered a compromise choice between Flexner and Reed.

"He is someone who will be a good prosecutor despite the fact that he has been a defense counsel for years," said one knowledgeable source. "He's smart, fast and disciplined."

One source said that as of a week ago, Litvack was the leading contender for the post, although the Justice Department has not yet forwarded Civiletti's choice for the antitrust job to the White House.

But despite the competition for the antitrust job, Shenefield, who has run the antitrust division since Carter took office in 1977, has yet to be formally nominated for the position he is temporarily holding. One official even suggested that he might steer the antitrust division through Carter's entire first term, which ends in 16 months.

Several officials said the administration had hoped to send up a package of nominations late this past summer for the Justice Department vacancies, including the Michael J. Egan (associate attorney general), Shenefield and Civiletti slots. Civiletti headed the department's criminal division before becoming attorney general.