By most accounts, Tiffany Dionna Jackson and David John Head were a couple of teenagers in love. Both 17, they had been dating for more than three years, they danced together at the prom and they doted on their baby daughter, Diamond.

So neighbors and friends were left shaking their heads yesterday when Prince George's County police arrested Head and two of his friends and charged them with murdering Jackson, who was fatally shot Monday. She had been taking a late-morning nap in her bedroom, her 11-month-old daughter curled up nearby.

Police said Head planned the shooting with the help of two buddies: James McKinley Benton, 18, of the 1000 block of Glen Willow Drive in Capitol Heights, and Kyle Joseph Bruce, 18, of the 3700 block of 65th Avenue in Landover.

Investigators said the three suspects drove to Jackson's house in the 6900 block of Standish Drive, just outside the town limits of Landover Hills, shortly after 11 a.m. Monday. As Head and Bruce remained in the car, police said, Benton walked up to the house, pushed his way in when a child answered the front door and went directly to the back bedroom where Jackson was sleeping.

Pausing only to wake Jackson up, Benton shot the young mother at point-blank range with a handgun, police said. The baby was unhurt, as were three 9-year-old children who were in the house at the time, police said.

Police arrested Head and Bruce yesterday morning and Benton yesterday afternoon. All three faced first-degree murder charges and were undergoing questioning in police custody last night. Police said they recovered the murder weapon but declined to give details.

Prince George's County District Court records show that Benton was arrested on July 27 and charged with two counts of theft. He was released and is scheduled to go to trial Oct. 27. Benton was indicted in December on charges of armed robbery, burglary and grand theft, but was found not guilty on April 23 after a trial, according to Prince George's County Circuit Court records.

Police said Jackson and Head had recently had some squabbles and may have talked about breaking up. But other than that, police were at a loss to come up with a motive for the crime.

"We're looking at whether there was a falling out of the relationship, which is probably where this is going to go," said Prince George's Police Chief John S. Farrell. "But there is no reason--there can't be a reason--to justify any of this. How do you make sense of something like this?"

Farrell declined to describe further the role that each of the suspects allegedly played in the crime or to talk about evidence that led to the arrests. But he said each of the teenagers knew each other.

"We have evidence that all three of them were part and parcel in the planning of this," he said. "The boyfriend, we believe, was intimately involved in causing the murder to take place."

Neighbors said they were stunned by the news that Head had been arrested. They said he appeared at Jackson's house shortly after the shooting and was seen crying and grieving with her relatives.

"I can't believe he'd put his daughter through all that," said Brandon Hall, 18, who lives around the block and knew the couple. "Kill his baby's mother? That's screwed up. That's pointless."

No one was at Jackson's home yesterday on Standish Drive, where she lived with her mother, daughter and two younger brothers. But friends put together a makeshift shrine of stuffed animals, snapshots and written testimonials on the windshield of a station wagon parked in front of the house.

One of the photos showed Jackson and Head sitting relaxing together on a porch. Another showed them dressed up at the prom, locked in an embrace.

Jackson graduated in June from Parkdale High School in Riverdale and worked full time at a Taco Bell in Lanham, usually taking the late shift so she could spend the day with her daughter. Co-workers said she was saving money to go to college.

"You were one of the most intelligent, young successful black mothers and one of the best friends I ever had," read one unsigned card posted at the shrine. "Ever since Diamond, you made a dramatic change for the better. You graduated, kept a job and are a damn good mother."

Staff researcher Bobbye Pratt contributed to this report.

CAPTION: At left, stuffed animals and photographs on a car outside the 6900 block of Standish Drive in Landover Hills memorialize Tiffany Dionna Jackson, 17, above.