FBI Director William Sessions said Wednesday he is suspending three bureau employees and censuring three others for mishandling a terrorism investigation into political opponents of the Reagan administration's Central American policy.

"I am disciplining these individuals because of the managerial or supervisory inadequacies displayed by them" during a bureau investigation of the Committee in Solidarity with the People of El Salvador, Sessions told a congressional committee."The mistakes in judgment that took place during the CISPES investigation were serious ones, and I cannot emphasize too strongly my firm conviction that there is no place for such mistakes in the work of the FBI," Session told the Senate Intelligence Committee.

Sessions became director of the FBI in November. He disciplined the agents for actions taken from March 1983 to June 1985, when William Webster was the FBI's boss. Webster now heads the CIA.

Sessions said the six employees work variously at the supervisory, unit chief and section chief levels and that they include personnel from FBI headquarters in Washington as well as at FBI field offices.

Three of the employees, whom he did not name, were placed on probation and suspended without pay for 14 days. The other three were formally censured, he said, with reprimands being placed in the employees' files that could affect their chances for promotions.

The director said a seventh employee, "whose performance would in all likelihood have merited dismissal," has resigned. That was an apparent reference to an agent who quit in 1984 after acknowleding he had given classified documents to an informant in the CISPES case.

Sessions testified that the investigation was launched in March 1983, based largely on information provided by informer Frank Varelli. The probe concentrated on the organization's headquarters in Washington, its Dallas chapter and on a handful of other CISPES chapters.

The investigation was prompted by tips that key members of CISPES were involved in covertly furnishing funds and material to a leftist group seeking to seize power in El Salvador.

Varelli told the bureau in 1984 that CISPES members were plotting to assassinate President Reagan and disrupt the Republican National Convention in Dallas.

FBI officials now say Varelli supplied them with disinformation and misinformation. He stopped working for the bureau in 1984 and filed suit in 1986 to get some back pay he felt he was owed.

"The major problem came when the scope of the investigation was unnecessarily broadened in October 1983," Sessions testified.

"The broadening of the investigation in essence directed all field offices to regard each CISPES chapter, wherever located, as a proper subject of the investigation."

Sessions said that, based on the available documentation, "there was no reason to believe that all CISPES members nationwide knew of or had any involvement in support of El Salvadoran or U.S. terrorists. Thus, there was no reason to expand the investigation so widely."

The FBI director said the decision to broaden the probe unnecessarily "was made at a comparatively low level and . . . supervisory personnel at FBI headquarters who should have reviewed and analyzed this decision and appreciated its significance failed to do so."

Sessions said there is no evidence that the White House or any other government agency or outside private group attempted to influence the CISPES probe. He said there is no evidence the probe was politically motivated or director or that the conduct by the six FBI employees who are being disciplined was illegal or motivated by any improper purpose.

The CISPES probe involved surveillance, undercover attendance at meetings, interviews with organization members and checks of telephone and utility company records and records of license and credit bureaus, as well as law enforcement agency record checks.