FBI Director William H. Webster announced the appointment yesterday of three top-level assistants in a move to improve the chain of command and reduce the clutter of decisions requiring his attention.

He said he wanted to get away from the "pyramid organization" of the J. Edgar Hoover era, when FBI officials were accustomed to passing the buck up to the director.

Designating them as sort of "group vice presidents," Webster named Lee Colwell, 45, as executive assistant director in charge of adminisration and Donald W. Moore Jr., 49, as executive assistant director in charge of law enforcement services.

The positions are new, replacing the old pyramid arrangement of an associate director who reported to the director and who in turn had two special assistants, each with a deputy, reporting to him.

The three executive assistants Webster named, all FBI veterans, will report to him directly and will be expected to make more decisions on their own and to help in the shaping of FBI policy.

Webster told reporters at a press conference that the new setup should give him "a much tighter organization that I can work with on a day-to-day basis." He said he was also going to try "a little cross-fertilization" and have the three men rotate assignments within six months to a year.

All three are "leading candidates" for the post of associate FBI director, which Webster said he was leaving vacant for now. It was last filled by James G. Adams, a longtime Hoover aide who retired in April.

Colwell joined the FBI in 1961, spent 17 years in the New York field office, primarily on foreign intelligence work, and has been in charge of the FBI's public affairs office. He will retain that job in addition to his new duties.

Moore started with the FBI as a clerk in 1949, has been special agent in charge of the Richmond, New Orleans and Washington, D.C., field office, and became assistant director in charge of the criminal investigative division 2 1/2 years ago.

In the shuffle, John H. Otto, who has been in charge of the FBI's Chicago office, will replace Colwell as head of the planning and inspection division. Francis M. Mullen Jr., a deputy assistant director, will succeed Moore as head of the criminal investigative division. Oliver B. Revell, who has been in charge of the FBI's Oklahoma City office, will replace Mullen.

While his new top aides will be expected to make "substantive decisions on more things," Webster emphasized that he still intends to direct the FBI."I'm reluctant to become chairman of the board," he said, "until it's proven to me that that's what I should do."

Responding to questions on other matters at the hour-long news conference, Webster said:

The FBI would prefer that its employes not "live in sin" with members of the opposite sex but won't do much about it unless their conduct becomes "notorious" and sets tongues wagging. "We're not going to be Big Brother to our employes," he said, "but we do expect them to be an example in their community."

Such permissiveness notwithstanding, the bureau still has a policy of not hiring practicing homosexuals and will adhere to it unless the courts strike it down. "Our position is that homosexual conduct -- and I'm not talking about homosexual preference, i'm talking about conduct -- is a factor to be considered when it comes to employment in law enforcement and we consider it a significant factor." CAPTION: Pictures 1, 2 and 3, HOMER H. BOYNTON, and DONALD W. MOORE, JR., and Lee COLWELL . . . new executive assistant, all FBI veterans, are expected to provide "a much tighter organization."