Three persons, two of them juveniles, were arrested Monday night in connection with the fatal shooting of a Federal Trade Commission economist and the wounding of his wife at their Parkfairfax town house Jan. 2, Alexandria police reported yesterday.

One of the juveniles, a 16-year-old, is charged with firing the shots that killed Steven Marston and wounded his wife, Martha F. Riche, in the chest.

Marston had just returned to their home at 3603 Greenway Place, and the couple was preparing to move from their rented house to a new home in another Washington suburb when the shooting occurred about 9 p.m. Riche, who ran screaming for help to a neighbor's house, was reported in fair condition yesterday at Alexandria Hospital.

The shootings occurred during a robbery attempt, and the couple's assailant may have followed Marston in through an open door, police said at the time. Neither police nor Alexandria prosecutor John E. Kloch would elaborate on the circumstances yesterday or say what led to Monday night's arrests.

All three of the suspects are from Alexandria, police said.

The 16-year-old was being held in lieu of $42,500 bond at the Northern Virginia Juvenile Detention Center on charges of murder, malicious wounding, robbery and burglary while armed with a deadly weapon, grand larceny and being armed during the commission of a felony.

Boggess Steven Davis, 20, of 421 N. Payne St., Alexandria, was being held in lieu of $20,000 bond at the city jail on a charge of robbery and burglary while armed with a deadly weapon.

The second juvenile, a 17-year-old, was charged with robbery and burglary while armed with a deadly weapon and held in lieu of $20,000 bond at the Prince William County jail because of crowding at city detention facilities.

The shootings stunned residents of Parkfairfax, a quiet community of townhouses not far from busy Shirley Highway.

Marston, 36, a former assistant economics professor at Cornell University, took a job in the FTC's bureau of economics last summer. His wife, 44, had recently been named editor of a new monthly newsletter on statistics, The Numbers News.

Ron Bond, deputy director of Marston's FTC bureau, said yesterday that friends and colleagues donated about $800 for a reward to hasten the apprehension of those responsible for the shootings.

Andy Strenio, assistant director of the FTC's bureau of consumer protection, said it will be up to Marston's wife to decide how to use the money. He added, "you couldn't work with Steve long and not become his friend. The only thing that can be done now is to follow the judicial process and see that justice is done. None of this will bring Steve back."