A man was fatally stabbed in the Bronx on Monday in an attack that unfolded about the same time a deranged man’s unrelated stabbing spree across Manhattan killed three people during a nearly 3-hour rampage, cops said. In the Bronx case, cops responded to a 911 call about an assault around 11:42 a.m. — less than an hour after the Manhattan stabber attacked his final victim  — discovered a 48-year-old man with stab wounds to his chest in front of a building at the intersection of Marion Ave. and E 194th St., police said. Medics transported the victim, identified as Nestor Padilla, 48, to St. Barnabas Hospital, but he could not be saved. Padilla lived in the Bronx about two miles away from where he was attacked, according to police. It was not immediately known what Padilla was doing in the area, or what triggered the deadly assault. There were no immediate arrests as police continue the investigation. cops said.