Two men were shot and one was seriously injured early Wednesday morning in Quincy, in an attack that police suspect was not random. According to police, a taxi pulled up near Arlington and Brook streets at 3:45 a.m. to let out four passengers. As they were paying the fare, a person came up and fired several shots. A 19-year-old Quincy resident was shot in the stomach and is in serious condition at Boston Medical Center, police said. A 22-year-old was hit in the elbow and was treated at Quincy Medical Center and released. The two other passengers were uninjured. Captain John Dougan said the attack did not appear to be random, but he did not elaborate.