The other juvenile, and the third injured person, who was 18 years old, suffered wounds that were not life-threatening, according to the spokesman.
A fourth person, a juvenile who was not injured, was taken in to custody, the spokesman said.
All four were males, according to the spokesman. No ages were given for the three juveniles.
The four were seen leaving a train together at the Branch Avenue station, Metro said. After they entered the parking lot, a dispute broke out, along with the shooting.
The nature of the dispute could not be learned.
The Branch Avenue station is south of Suitland Parkway in the Marlow Heights/Camp Springs area.
Metro said it is continuing to investigate.