A problem with a furnace is considered the likely cause of carbon monoxide poisoning that killed three people and sickened a fourth in a home in Plaistow Tuesday. State Fire Marshall J. William Degnan said the fourth victim was taken to Massachusetts General Hospital after firefighters were called to the Plaistow home at 11:30 a.m. His condition was not immediately released. Degnan said the home had a carbon monoxide detector, but it did not have batteries. He said the investigation is focused on the propane-fired furnace. All the victims were adults. One woman and two men were unresponsive when emergency crews arrived, said Plaistow fire officials.