A three-judge federal court appointed a special prosecutor yesterday to investigate allegations that White House chief of staff Hamilton Jordan used cocaine.

Acting in response to a report by Attorney General Benjamin R. Civiletti, the judges named New York lawyer Arthur H. Christy, 56, to conduct the inquiry outside regular Justice Department channels.

His mandate appeared to be somewhat broader than the one Ceviletti recommended.

A former U.S. attorney, Christy was instructed by the judges to investigate charges that Jordan used cocaine on a June 27, 1978, visit to a New York discotheque, "and any other related or relevant allegations" that Jordan violated the federal law against possession of such drugs.

Civiletti had recommended that the inquiry be limited to the allegations about Jordan's visit to the discotheque, Studio 54, and to the question of whether false statements had been made to the Justice Department during the preliminary FBI investigation.

In his report to the court, the attorney general also made plain that he was recommending appointment of a special prosecutor under the 1978 Ethics in Government Act only because of the strict standards that law imposes in weighing allegations against high-ranking government officials.

Civiletti said that in his judgment, the preliminary FBI investigation of the Studio 54 case left it "so unsubstantiated that prosecution is not warranted." The law, however, requires appointment of a special prosecutor unless the attorney general concludes that "the matter is so unsubstantiated that it warrants no further investigation."

The attorney general said he was unable to go that far.He noted that "information from a number of pertinent witnesses" could not be obtained during the preliminary investigation. Several wanted immunity from prosecution in return for their testimony.

In addition, sources said, there were several other potential witnesses at an Oct. 21, 1977, party in Beverly Hills, Calif., whom the FBI never contacted because it never learned their names during a preliminary investigation of that incident. Jordan was also publicly accused of having snorted cocaine at a nightclub that evening, but Civiletti, in his report, dismissed that allegation as completely "unsubstantiated."

Civiletti notified President Carter of the court's action in a 15-minute meeting at the White House yesterday morning. Carter immediately reaffirmed his faith in Jordan, who has denied ever using cocaine, and asked him to stay on the job.

"Hamilton Jordan is a young man in whose ability I have complete confidence," the president added later in the day, during a briefing on SALT II. He said he was "as confident that he [Jordan] would tell me the truth" as he was of his own wife and children.

Echoing the president, the attorney general said through his spokesman, Bob Smith, that "there ought to be no cloud on Mr. Jordan" because of the renewed unquiry.

In a brief statement, Jordan noted that "from the outset I have flatly denied the allegations made against me" and said he had cooperated fully in the investigation thus far. He said he intends "to cooperate fully in any further investigation," but was disappointed that the attorney general felt compelled to seek a special prosecutor "because of his inability under the ethics act to obtain testimony under oath."

Christy made plain to reporters in New York, after his appointment, that he did not consider himself confined to the Studio 54 incident.

It is not limited to New York," he said of the investigation. "The [court's] mandate has no territorial limitations."

A lawyer who gained a reputation as a tough, young prosecutor in the 1950s, Christy assisted in the income tax prosecution of mobster Frank Costello and directed the investigation of the acid-throwing attack on newspaper columnist Victor Riesel, which resulted in the indictment of mobster Johnny Dio. Christy also successfully prosecuted organized crime boss Vito Genovese and worked on the prosecution of Rep. Adam Clayton Powell (D-N.Y.).

A Republican, Christy was named U.S. attorney for the southern district of New York in 1958, at age 34, but he did not win reappointment after 10 months because he was deemed too young, and he went into private practice.

The three members of the special court, U.S. Circuit Judges Roger Robb of Washington, D.C., J. Edward Lumbard of Bridgeport, Conn., and Lewis R. Morgan of Newman, Ga., selected Christy after meeting in New York Monday and Tuesday.

Lumbard said Christy is the first person to be sworn in as a special prosecutor under the 1978 ethics act.

The controversy involving Jordan began Aug. 22 when attorneys for the owners of Studio 54, who were under indictment on income tax evasion charges, told the Justice Department that Jordan had snorted cocaine at the nightclub the year before.

One of the club's owners, Steve Rubell, said he saw Jordan take "a hit of cocaine" from a drug dealer who frequented the discotheque, John (Johnny C) Conaghan. Conaghan said he administeed the drug to someone at Rubel's behest on the night in question, but could not independently identify the person as Jordan.