Two of the robberies have taken place in Queens.
In those cases, the gunman got off the train at the 80th Street station.
The third incident happened in Brooklyn, where the suspect fled the train at the Shepherd Avenue station.
All the robberies happened between August 1 and August 19.
The suspect is described as a black male between 20 and 25 years old and stands 5-foot-10 to 6-feet tall.
Anyone with information in regards is asked to call Crime Stoppers at 1-800-577-TIPS (8477).
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