A maniac slashed three elderly people with a machete in an unprovoked attack and was fatally shot by cops at Grand Central Terminal Saturday morning — causing mayhem that sent commuters fleeing the iconic station, police and witnesses said. An 85-year-old man and a 65-year-old man were slashed in their heads on the 4, 5, 6 subway platform at at 9:50 a.m., police said. A 70-year-old woman was also slashed, but it wasn’t clear where on her body, cops said. Two NYPD detectives confronted the 44-year-old and told him to drop the machete. He ignored their commands and was shot...