A Prince George's County couple abruptly abandoned a lawsuit yesterday which claimed that two county detectives had wrongfully accused them of murdering their own children in a fire.

Robert and Margie Raffety - in the sudden about-face in federal court in Baltimore - also dropped their claim that a county fire investigator was negligent in his arson determination after the blaze.

The Faffety's attorney acknowledged in the courtroom yesterday that all three men had acted reasonably in performing their duties. The dismissal decision apparently turned on the testimony Thursday of the fire investigator, which the Raffetys believed they could not refute.

THe couple's three children, aged 5, 3, and 4 months, were killed in the fire Jan. 21, 1975, in their Mount Rainier home.

The fire was declared an arson, but no arrests have been made.

The Raffetys, who are now separated, had said in a $250,000 damage suit that they were held for more than 10 hours of intensive questioning just hours after the blaze was extinguished.

The jury trial on the suit began Tuesday with a barrage of charges from the Raffety's lawyer and a flurry of denials from the defense. it ended yesterday with a clipped, precisely-worded statement read by the couple's attorney, Jayson Amster.

"The plaintiffs acknowledge that under the facts and circumstances, plus the testimony in evidence up to this point, the fire investigator Mr. (David) Malberg, had reasonable grounds to conclude that there had been an arson . . . Ther police officers, Mr. (David) Hatfield and Mr (R. Tony) Tucker, acted reasonably under those circumstances . . ."

Raffety, in an interview later yesterday at his new home in Upper Marlboro, said they decided to drop the suit after Malberg testified about what he had found in the couple's fire-ravaged Mount Rainier home after the blaze.

"Lt. Malberg was an exceptionally good witness, well-prepared," Raffety said.

"He had the jury eating out of his hand."

Raffety, a District of Columbia policeman, said his lawyer would not have been able to refute the basic shape" of Malberg's testimony because the Raffetys' expert witness on fires, a Denver chemist, had not been at the scene of the fire.

Raffety insisted there were "discrepancies" in Malberg's testimony, but going after them would have been just "nitpicking."

"I didn't like giving up, but it would have been pointless to go on . . . My wife couldn't have been subjected to going through that again. She didn't think she could take it. Neither did I."

Michael Connaughton, an attorney for the detectives, agreed that Malberg's testimony had done the trick.

"After that I think the Raffetys were faced with the fact that they could not prove this was not an arson," Connaughton said after the trial.

Cpt. Tucker said he felt "vindicated and relieved" after the "long, trying experience."

And County Executive Winfield M. Kelly Jr. issued a statement announcing "that what happened in court today illustrates what we have always known - that these men are professional."

Raffety, sitting in his family room near three poster-size photographs of his late children, explained that as a policemen he had at first been reluctant to bring the lawsuit.

And now that it's all over?

"I feel no malice toward them," Raffety said.

"Malberg, Hatfield and Tucker did their job to the best of their ability."