Published Wednesday, 17 June 2009
Another nine possible cases are also under investigation.
The latest cases, two men and a woman, travelled outside the UK but are not related.
They developed mild symptoms after returning home.
They are all being treated with antivirals and those who had been in close contact with them are being contacted by the Public Health Agency.
Dr Carolyn Harper, director of public health at the Public Health Agency, said: "We have been expecting further cases of swine flu. People should not be alarmed but should continue to be alert.
"Systems are in place to respond to more cases over the next few weeks."
She said to slow the spread of the flu in the province it was important people who developed symptoms within a week of returning from flu-affected countries stayed at home and contacted their GP.
The PHA said the other nine cases in Northern Ireland had either recovered or were recovering well.