A witness to an attempted robbery in Antioch helped police arrest three would-be thieves Wednesday evening, police said today.At about 8:30 p.m., two males wearing ski masks and armed with handguns approached a woman in the Walmart parking lot at 4893 Lone Tree Way and told her to hand over her purse,police said.Several customers witnessed the confrontation and intervened, scaring the robbers away, police said.One witness watched as the pair got into a car driven by a third person. The witness took down the license plate number and followed the car at a distance while updating police, police said.Police responded and arrested all three suspects. The two people who were wearing ski masks turned out to be two juveniles, ages 17 and 14, police said. The driver has been identified as 18-year-old John Mixon. All three are Antioch residents.Inside the car, officers found two ski masks and two Airsoft pistols, police said. The juveniles were booked into juvenile hall and Mixon was sent to the Martinez Detention Facility.