The National Maritime Authority (AMN) released a statement after the Danish-flagged pleasure boat capsized about a kilometer off one of the beaches at Santa Cruz, in the municipality of Torres Vedras, some 60 kilometers (37 miles) north of Lisbon.
According to Portuguese Navy spokesperson Jose Sousa Luis, the victims—two men and a woman, whose ages and nationalities were not immediately known—were discovered dead on the sand close to the boat's wreckage and had not worn life jackets.
He said that three individuals were on board when the boat departed the port of Peniche on Friday morning, which is around thirty kilometers north of Santa Cruz.
Storm Ciaran did not directly strike Portugal, yet it has killed twelve people in other parts of Europe.
The nation's central and northern coastal districts, meanwhile, are under a red alert from the National Meteorological Institute (IPMA) due to extremely choppy seas with waves as high as seven meters (23 feet).
According to the IPMA, a new storm with waves as high as nine meters was predicted to pass off Portugal on Saturday and Sunday. The storm was given the name Domingos by the Spanish meteorological organization.