People have been urged to help contain the spread of the virus at the Conquest Hospital in Hastings by not visiting friends or relatives in hospital if they have symptoms of the vomiting bug.
Tina Lloyd, assistant director of infection prevention and control, said stringent measures had been brought in.
The affected wards are Newington, Baird and MacDonald.
Ms Lloyd said East Sussex Healthcare NHS trust would prefer that people did not visit the affected wards, but she said if their visit was essential they should restrict the time of the visit and wash hands before and afterwards.
She urged people to follow good hygiene measures.
Three wards at a Sussex hospital have been closed to new admissions following an outbreak of norovirus.