Sudbury police have arrested three men who robbed a grocery store in Chelmsford. The incident started out like a shoplifting case, with one man putting food in a backpack then leaving the store without paying. Security ran after him but stopped when the man pulled a knife on them and then hopped into a getaway car. Police found the two accomplices a few hours later and arrested the backpack man the next day. All three are charged with robbery with a weapon while the man who took the food faces additional charges including two counts of uttering threats.