Police shot and lightly wounded the gunman, who is now in custody. Police said it was not a terrorist incident.
A major police operation is under way in the northern suburb of Unterföhring, and rail services have been suspended.
Police said there was no further danger to the public.
The two wounded bystanders are being treated in hospital, while the policewoman is fighting for her life.
Police said the shooting happened when police intervened in a scuffle between several people at the station, one of whom had a knife.
The man fired at least five shots. He was lightly injured when police fired back, and is now being questioned.
German n-tv news says he is a 37-year-old German citizen and a Munich resident.
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The officer’s injuries were considered life-threatening, while those to the two bystanders shot at the Unterfoehring station were less serious, the Munich police spokesman Marcus da Gloria Martins said.
The suspect was seriously injured after being shot by police, he said.
“There was one suspect and we’ve got him,” Da Gloria Martins told reporters at the scene.
Authorities did not believe the incident was terrorism-related, and that the suspect appeared to have acted out of “personal” reasons and not with political or religious motivations, Da Gloria Martins said.
Police had been called to the subway station after receiving reports of an altercation between passengers on a train. When officers arrived the suspect tried to push them on to the tracks, Da Gloria Martins said.
The suspect then grabbed the officer’s gun and fired several shots before he was shot himself and taken into custody.
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