Dorset Police received a report at 7.16am on Wednesday regarding three residents two men, aged 74 and 91 and an 86-year-old woman - who were found dead at Gainsborough Care Home in Ulwell Road, Swanage.
Seven people were taken to hospital as a precaution and were not in a serious condition, the force has said.
Other residents from the address were evacuated and have been moved to other homes until they can be returned to the Swanage care home.
The deaths are being treated as unexplained and detectives are still exploring possible carbon monoxide poisoning as the primary line of inquiry. There is no wider risk to the community, the force said.
A 60-year-old woman arrested on suspicion of manslaughter has since been released under investigation.
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