An off-duty NYPD cop opened fire on three alleged thieves on a Manhattan street when she thought they were reaching for a weapon on Tuesday, cops and sources said. While inside her own car, she spotted three men trying to steal a catalytic converter from another parked vehicle, according to police.The off-duty cop then began filming the dodgy trio – which they noticed and tried to block her vehicle, the sources said. Then at least one of the three began reaching into their waistband to grab what appeared to be a weapon, according to the sources. In turn, the officer fired off one round, but no one immediately appeared to have been struck, cops said.