The first death occurred about 12:35 p.m. on Sunday when an American soldier who was accompanying United States officials visiting Baghdad University was fatally shot by an unidentified gunman, witnesses and American officials said.

The next death occurred at 9:30 Sunday night when a soldier from the First Armored Division was killed while chasing two Iraqi gunmen. One of the Iraqi gunmen was killed and one was wounded, said a military spokesman, Specialist Lorente Giovanni.

The third soldier was killed at 1 this morning while he was on patrol in a Baghdad neighborhood when an explosive device struck his vehicle, Specialist Giovanni said. 

At the university, several students who said they had witnessed the shooting, outside a cafeteria near the engineering school, said the gunman had pulled out a pistol and fired a single shot at point-blank range into the soldier's head. An American official said the soldier had apparently just left the cafeteria after buying a drink.

The gunman fled, disappearing into the crowd of students, witnesses said. The soldier, whose name was not released, was evacuated to a military hospital, where he later died, military officials said.

The three deaths represent the 26th, 27th and 28th American soldiers to be killed in hostile action since May 1, when President Bush said major fighting had ended.

The shooting at the university came a day after seven Iraqi police recruits were killed and more than 70 other people were wounded when an explosive device attached to a utility pole near a police station in Ramadi was detonated during the graduation of the first class of the new American-trained Iraqi police force. There were several other reports of hostile actions against American forces this weekend, and a British journalist was shot and killed in Baghdad on Saturday.

The attack on Sunday afternoon raised questions about whether the movements of American officials were being tracked by Iraqi insurgents. An American official said over the weekend that some Chevy Suburban vehicles that ferry Americans around the city under heavy security have been shot at in the last month.

The shooting at the main campus of Baghdad University, in southern Baghdad, came as officials with the Coalition Provisional Authority, the United States-led civilian administration of postwar Iraq, were visiting officials at the university. The university's library contains temporary offices of the Iraqi Ministry of Higher Education.

Military officials have described recent attacks as the isolated work of desperate Iraqis connected to the toppled government of Saddam Hussein. On Sunday, however, the chairman of the Joint Chiefs of Staff, Gen. Richard B. Myers, said officials were examining whether the attacks by remnants of Mr. Hussein's Baath Party were becoming better organized. That "remains to be seen," General Myers said on "Fox News Sunday."

"They certainly weren't at first," he said, "and we're still evaluating that to see if there is any kind of coordination that goes beyond just local coordination, that it might be broader than that."

Military officials in Baghdad said on Sunday that a seven-day effort to root out Iraqi resistance and paramilitary activity in central Iraq had resulted in the detention of 282 people and the seizure of weapons that included 217 rocket-propelled grenades and 96 AK-47 rifles.

Several university students who saw the shooting said the gunman did not appear to be a student from the university. But they emphasized that the university, which is spread out over several campuses in Baghdad, has more than 60,000 students, making it impossible to know for sure whether he was affiliated with it.

One witness, Thair Khudair, a 22-year-old civil engineering student, said two American soldiers were standing next to the cafeteria when the gunman pulled out a pistol and shot one of them. 

"We just heard one shot," Mr. Khudair said. "I was sitting in the cafeteria, and through the glass I saw it." 

Although in other parts of Baghdad and central Iraq many Iraqis contend that they have been mistreated by American forces, Mr. Khudair and several other students said they were shocked by the attack on their campus.

"No student would shoot an American," Mr. Khudair said.

One allied official said on Sunday night that he did not know the gunman's motivation, or whether the attack was linked to the visit by allied officials to the campus.

American troops cordoned off the campus and searched students, according to students who said they had been detained for up two and a half hours before they were allowed to leave. "A friend of mine saw a man stand in front of the Americans and take a nine-millimeter pistol and shoot him and run away," said Anwar Sabah, a 23-year-old engineering student. "I saw the guy lying on the floor and blood coming from his head."

Witnesses said the soldier had been shot in the head at close range. One week ago, the chief civilian administrator for Iraq, L. Paul Bremer III, said recent attacks on American soldiers, including one in which a soldier was shot in the head at a Baghdad market nine days ago, suggested "a clear understanding of how body armor works."

The soldier was with the 352nd Civil Affairs Command in Baghdad, which is attached to the Army's First Armored Division, said Maj. Sean Gibson of the United States Marines, a spokesman for allied forces.

Major Gibson said an Iraqi was shot and killed by American soldiers with the Third Armored Cavalry Regiment on Saturday night in Ramadi after the Iraqi charged a military checkpoint in his vehicle. 

According to The Associated Press, two Iraqis in a pickup truck also were killed Saturday night in Baghdad as they charged troops and fired a rocket-propelled grenade.