All three victims were of Middle Eastern descent. Their stores are within a 5-mile radius, and none of them have surveillance cameras.
The latest shooting death came Friday, when 78-year-old Rahmatollah Vahidipour was killed at his store, She She, in Brooklyn's Flatbush section. He suffered gunshot wounds to the head and torso, police said.
Police said the same gun was used in the deaths of two other Brooklyn shopkeepers over the summer.
On July 6, clothing store owner Mohamed Gebeli was killed inside Valentino Fashion in Bay Ridge. On Aug. 2, Isaac Kadare was found dead at his Amazing 99 Cents Deal shop in Bensonhurst.
Authorities have said that ballistics evidence connected the weapon used in the first two deaths. Police say they don't know the motive behind any of the killings.
A call on Sunday to Vahidipour's home in Great Neck wasn't answered.
Authorities are offering rewards of $22,000 for information leading to an arrest and conviction in the three homicides.