Amos J. Wells III confessed to shooting his pregnant girlfriend, Chanice Reed, 22, her mother, Annette Reed, 39, and her brother, Eddie McCuin, 10, at their home in the Fort Worth suburb of Forest Hill, the Fort Worth Star-Telegram reported Tuesday.
The victims later died at area hospitals.
Investigators believe the Monday night shootings were prompted by an argument between Wells, his girlfriend and her mother, Fort Worth police Sgt. Joe Loughman said.
Wells fled the house after the shooting, police said, and turned himself in to police later that evening.
Forest Hill Police Chief Dan Dennis said Wells "walked into our lobby and said we needed to arrest him because he 'did something bad.'"
Wells has twice been accused of domestic abuse, but neither case involved Reed, court documents indicated.