Three witnesses who said they knew both murder suspect Wayne B. Williams and 15-year-old Joseph (JoJo) Bell, who was slain early last year, testified today they saw the two together on several occasions, contradicting Williams' public statements.

The jury in Williams' trial on two murder charges has heard a tape recording of the defendant's June 4 press conference in which he claimed not to know any of the 28 victims whose deaths were being investigated by a special police task force and the FBI.

Kent Hindsman, a 24-year-old song writer now working as a waiter, said he saw Bell and Williams during a recording session at an Atlanta studio in 1981. He said Williams drove him and Bell downtown after the session in a white station wagon, and that Williams said he had signed Bell to a contract as a singer.

Hindsman also said he accompanied Williams to the Omni, a downtown shopping and entertainment complex, for what Williams called "star-gazing: looking for potential stars."

When he told Williams he thought it was "a shame the kids were being killed," during Atlanta's string of murders, Hindsman said Williams replied, "point blank-- 'They ought to keep their damn a---- at home.' "

Hindsman said that when he first met Williams the 23-year-old aspiring music promoter asked if he was a "bum" or "gay" or was "out on the streets all the time."

During one recording session, a woman sitting between Hindsman and Williams passed a note to Hindsman apparently written by Williams, saying, "I could be a president, I could be a mayor, or I could be a killer," Hindsman testified.

When Hindsman replied, "Who would write such a thing?" the woman snatched the note away from him, he said.

Hindsman also said Williams claimed to have been thrown out of the Air Force "for flying an airplane under a bridge," and said he had a black belt in karate and had had six months of law enforcement training. All of these claims are believed to be false.

Two brothers, John Laster, 15, and Lugene Laster, 21, testified that Williams visited their grandmother's house to interview John before an audition in which he sang for Williams.

John Laster said he had known JoJo Bell for two years and often played basketball with him. One day while walking home from high school basketball practice, Laster said, he saw Williams driving slowly toward Bell in a white station wagon.

Lugene Laster testified that after playing basketball with Bell at an elementary school playground one day last year, he saw Bell get into a sky-blue station wagon with Williams. The older Laster said he got into his car and followed the station wagon until it entered an interstate highway.

Hindsman, testifying after the Laster brothers, provided a clue that may resolve what appeared to be conflicting testimony about Williams' cars.

Hindsman said he had seen Williams driving three different station wagons at various times: one white, one blue and one green. Witnesses have placed Williams with three of the murder victims in cars of each color.

Bell was last seen alive March 2, 1981. He died of asphyxiation, and his body, clad only in torn underwear, was found in a river southeast of Atlanta on April 19, 1981.

Bell's case is among 10 that Judge Clarence Cooper ruled prosecutors may introduce to show that the deaths of Jimmy Ray Payne, 21, and Nathaniel Cater, 27--which Williams is accused of--are part of a pattern.

The cases of six young blacks have been introduced so far this week.

Two former investigators from the sheriff's office of suburban Rockdale County testified today that they had seen Williams near the site where the body of 15-year-old Terry Pue, another of the 10 allegedly related victims, was found on Jan. 23, 1981. They said Williams told them he was a free-lance photographer, although Williams has claimed he did not cover any of the Atlanta killings as a photographer.