Twenty one patients have now died with coronavirus following an outbreak at a hospital.
To date 127 cases have been linked to an outbreak at the Royal Glamorgan Hospital in Llantrisant, Rhondda Cynon Taf.
Two people have also died amid outbreaks at Prince Charles Hospital, and the Princess of Wales Hospital.
Cwm Taf Morgannwg Health Board said “immediate measures” had been taken to stop the virus spreading.
The health board said it was working with Public Health Wales to closely monitor the situation at the three hospitals.
As of Tuesday, 17 cases had been linked to an outbreak at Prince Charles Hospital, in Merthyr Tydfil, and one person had died.
Figures also show 15 cases have been linked to Princess of Wales Hospital, Bridgend, and one person has died.
Visits to the hospitals were suspended in September, following concerns in a rise in coronavirus cases in communities.
Rhondda Cynon Taf, Bridgend and Merthyr Tydfil, are all currently subject to local lockdown restrictions, with people banned from socialising indoors and from entering or leaving the areas without a “reasonable excuse”.
Medical Director for Cwm Taf Dr Nick Lyons said: “The safety of our patients and staff remains our first priority and immediate measures to contain the spread of the virus have been put in place.
“We are taking the outbreaks extremely seriously and the stringent and robust mitigating actions which have been taken across our sites are being closely observed.
“However given the nature of Coronavirus, there is an inevitable time delay in when we will see the positive impact of these measures. “
A helpline has been set up for any relatives or people living in the communities who have concerns, which can be reached on 01685 726464.
Dr Lyons said infection rates in communities were “continuing to rise” and appealed to people to not break social distancing and lockdown rules.
“We remain grateful to all members of our community who are continuing to adhere to the guidance in order to help control this virus,” he said.