Five wards at Wrexham Maelor Hospital are closed to the public, with restrictions on another, after an outbreak on Friday.
Currently, 32 patients are affected by the winter vomiting bug.
Betsi Cadwaladr University Health Board is now investigating whether sickness at Chirk and Mold Community Hospitals is due to the same bug.
Investigations have been launched at two north Wales hospitals after an outbreak of norovirus in Wrexham.