The shooting was the second tragedy the Marine Corps faced this week. Seven members of the 2nd Marine Expeditionary Force were killed Monday when a mortar shell exploded in its firing tube during an exercise at Hawthorne Army Depot in Nevada. Eight others were injured.
Maxwell referenced the Nevada deaths in his remarks, choking up as he told reporters that chaplains at Quantico would be providing counseling for Marines and their families.
"I want to express my sincere condolences to the families, friends and fellow Marines of the three Marines we lost last night," Maxwell said. "Our thoughts and prayers are with them at this time. This is a tragic loss for our Marine Corps family."
Maxwell said he anticipates a "lengthy investigation" and did not expect the identities of the shooter or the victims to be released until at least Saturday.
Pentagon press secretary George Little said Defense Secretary Chuck Hagel was saddened.
"This tragedy, as well as the tragedy in Nevada earlier this week, took the lives of Marines who volunteered to serve their nation," Little said. "His heart and his prayers are with them and their families."
The Quantico base, which is 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 sentenced to 25 years in prison.