Chicago police are warning businesses on the North Side to be on alert after a series of armed robberies. Eyewitness News has obtained exclusive surveillance video of one of the brazen robberies in Ravenswood.Around 8 p.m. Wednesday night, it took just a couple minutes as the clerk at Windy City Liquors in the 4900-block of N. Damen was surrounded and manhandled with a gun pointed at his face while the cash register is raided.A surveillance camera captured three men with their faces covered and wearing hoodies. They stormed into the business, overwhelmed the clerk and ordered him to open the register. Then, he's dragged to the back of the store with one of the robbers waving a gun.For several agonizing seconds, his fate is unclear. He seems to plead with them with the gun trained on him the whole time. But after collecting all the cash, the thieves eventually leave and the clerk is left unhurt."It's terrible. I can't imagine how he felt. I'd be terrified... It seems to be happening more and more around here. People are too brazen and think they can do whatever they want," said Denise Smedick, a store customer.Police said that robbery was one of at least three Wednesday night. They all occurred within a couple miles and couple hours of each other. An Albany Park convenience store was one of the other businesses hit.In all three incidents, police said there were two or three men with their faces covered who pushed workers to the back of the store. Now, police are urging North Side business owners to be on alert.No one was injured in any of the three robberies. In all, the thieves made off with thousands of dollars in cash, alcohol, and cigarettes.