A third convenience store robbery in less than three weeks has Norwalk residents on edge.In all three incidents, police say the clerks were assaulted and the suspects escaped with cash. The first robbery took place at Rainbow News and Variety on Main Street last month. The second happened at the Shorehaven Mobil last week, according to police.On Sunday night, two men robbed the Mobil gas station on North Avenue and beat up the store clerk. Police say the men made off with lottery ticket money. The assaulted clerk was not taken to the hospital. While police say they believe the first two robberies are connected, they do not believe the same suspects are behind the third. Some area residents say they are frightened for their safety. The robbers from Sunday night's incident were captured on surveillance video. Anyone with information is urged to call Norwalk detectives at 203-854-3011.