Police evacuated 115 residents on Wednesday from the facility, whose air conditioning was cut by the storm.
Broward County Mayor Barbara Sharief said three were found dead at the nursing home in the city of Hollywood. Three others died in hospital.
Ten million people are still without power in Florida, Georgia and the Carolinas after Hurricane Irma.
The storm, which has claimed more than two dozen lives in the US, struck southwestern Florida on Sunday morning as a category four hurricane before weakening to a tropical depression on Monday.
