Taxi drivers across the borough are on edge following a string of cab robberies. Police say three livery cab drivers have been robbed and assaulted this month. The first hold-up happened on March 17 at East 88th Street and Seaview Avenue in Canarsie. Police say two men demanded money as they threatened the driver with a machine gun. A similar incident took place Saturday at 21st Avenue and 61st Street in Mapleton. Authorities say three men also robbed and assaulted a driver yesterday in Williamsburg. None of the drivers were seriously hurt, but the New York Federation of Taxi Drivers is offering a $1,000 reward for information leading to an arrest in any of the cases. Anyone with information is urged to call Crime Stoppers at 1-800-577-TIPS.Man wanted for armed livery cab robbery