Marine Col. David Maxwell, commander of Marine Base Quantico, told reporters during a Friday morning press conference that the dead Marines' identities would be withheld for the next 24 hours or until their next of kin was notified.
Maxwell said all three Marines were permanent personnel assigned to the Marine Corps' Officer Candidates School.
Authorities are still investigating and aren't speculating what possible motives might be.
"It's been a long night as we have begun to deal with the tremendous loss that happened last night, and on behalf of Marine Corps Base Quantico and the Officers Candidates School, I want to express my sincere condolences to the families, the friends and the Marines- of the three Marines that we lost last night," Maxwell told reporters. "Our thoughts and prayers are with them at this time. This is truly a tragic loss again for the Marine Corps."
Maxwell said authorities received a 911 call around 10:30 p.m. Thursday night. Maxwell also disputed earlier reports that the shooting led to a standoff between authorities and the suspect, who was barricaded in barracks at the base.
"That was not the situation this morning," Maxwell told reporters. "There was not a standoff or barricades."
Authorities entered the barracks early Friday and found the suspect dead of a self-inflicted gunshot wound along with a second victim. Base spokesman Lt. Agustin Solivan could not say what prompted authorities to enter the barracks, which are at the school.
Solivan said the shooting was isolated to the school and authorities were confident there were no other casualties. The base was put on lockdown after the shooting but the lockdown was lifted early Friday. During the lockdown, residents were warned over a loudspeaker to stay inside.
The base, which is about 37 miles south of Washington, is also home to the FBI's training academy.
In 2010, the base was one of several targets of an ex-Marine reservist who, during five nighttime shootings, fired on military targets including the Pentagon. Yonathan Melaku, on two separate occasions, fired at the National Museum of the Marine Corps in Quantico. No one was injured and Melaku was ultimately sentenced to 25 years in prison.