A statement from Dorset Police said officers were called at about 07:15 BST after three people were "found deceased" at the care home in Ulwell Road, Swanage.
A force spokesperson said the deaths were being treated as "unexplained" and an investigation has been launched.
"Inquiries are being carried out to establish the full circumstances and we are continuing to liaise with our partner agencies," they continued.
"The families of those who have died have been informed and the coroner has been notified."
Four other people have been taken to hospital as a precaution, they added.
