Around 10:15 p.m., the women, one of whom had a 1-year-old baby in the car, parked the silver Prius at a convenience store in the 2200 block of Gellert Boulevard, according to police.
When the women got out of the vehicle, the suspects exited a Dodge Durango, brandished a pistol and demanded their purses. The owner of the Toyota was pistol-whipped, police said.
After the suspects left, the car owner realized she had given them the keys to her car.
When she went back to the store parking lot, she saw her Prius and a red van being driven away, police said.
One of the victims memorized the Durango's license plate and police later found it at the owner's home in the Reston Court area with some of the stolen items inside.
None of the suspects were at the home.
Police are still investigating the case and asking anyone with information to call South San Francisco police at (650) 877-8900.