A Washington man convicted more than a year ago of three murders at a Northeast Washington fish market was acquitted yesterday of those charges after a retrial of the case before a jury that presumably was not aware of the earlier convictions.

Melvin Douglas Downing, 23, has been in jail for the 18 months since he was arrested on the murder charges and now will be released. Two other persons have been found guilty in the case, including one defendant who admitted he was the lookout man and who told investigators Downing was inside the market at the time of the murders.

Both codefendants - one a 17-year-old juvenile found guilty in a juvenile proceeding, who can be held until he is 21, and the other a 27-year-old man, who is serving a life sentence after his guilty plea - declined to testify at the second trial, and the government made no effort to compel their appearance.

The retrial was ordered by D.C. Superior Court Judge Sylvia Bacon after a juror at the first trial claimed he visited the scene of the crime while the jury was deliberating.

The juror claimed at a later hearing that he really had not visited the murder scene, but had said that to fellow jurors only to "save face" because he had been the lone holdout against conviction before changing his mind.

Bacon did not accept the juror's second version, and awarded Downing a new trial before a different judge.

Downing, who was arrested four days after the murders, testified during his retrial that he had been at the fish market at Florida Avenue and Orren Street NE earlier on the night of the murders but that he had left before the slayings occurred. He claimed the surviving witness accused him of the murders because he disliked Downing.

Downing was arrested after the surviving witness, Benjamin Washington, testified from the hospital that Downing participated in the crime.

Washington was one of four persons in the Berkley Farms market shortly after midnight on Feb. 7, 1976, when two men entered the store in an apparent robbery attempt.

Washington testified at the second trial that the two men used machetes and a pistol to murder the other persons in the store and also critically wounded him, saying they were leaving him for dead because he would bleed to death from his wounds.

Although he was not expected to live, Washington recovered and identified the persons he said were his assailants.

In addition to Washington's testimony, the government also produced other evidence at the second trial that it said linked Downing to the crimes. A machete believed to be used in the murders was found on a roof of his apartment building; gloves believed used in the crimes were found in a field next to his apartment, and the gun and clothing used in the crime were found near a female friend's home after an anonymous telephone tip.

Downing's attorney, Kenneth Mundy, argued that the evidence against his client was insufficient to convict him of murder, and that he was singled out for accusation by Washington.

Murdered at the fish market were Samuel Tillman Jr., 27; Raymond Washington, 27, a cousin of the wounded man, and Charles Edward Scott, 27. Tillman owned the store, Washington helped him operate it and Scott was a customer there.

An undetermined of money was taken during the robbery, but the codefendant who pleaded guilty said his amount of the take for serving as a lookout was $100. Police speculated that the men in the store were killed because they knew their attackers.

Downing had no previous convictions.