UPDATE via the Washington Post:
Chris Layman, a spokesman for the Pentagon Force Protection Agency, said the two people shot are Pentagon Force Protection Agency police officers. He was not certain of their condition. He said they have one person in custody.
ARLINGTON, Va. – FOX 5 has learned that three people have been injured in a shooting outside the entrance to the Pentagon Metro station.
Few details were immediately available, but three people who were injured in the shooting were taken to George Washington University Hospital’s emergency room, officials said. The Metro station was closed.
Metro officials confirm the shooting took place outside the entrance to the Metro station. An announcement was made to passengers for them not to exit at the Pentagon Metro entrance because it is closed.
Full story: 3 Shot Outside Pentagon Metro Station