Authorities say all three children attended the Washington County Agricultural Fair the week of Aug. 13 to Aug. 20.
They say one child has recovered, while two confirmed ill over the weekend are recuperating.
The cases are similar to past human infections with swine-origin H3N2 viruses. But the new strain also contains the H1N1 virus linked to the 2009 flu pandemic.
The state is investigating how the illness was transmitted. But health department spokeswoman Christine Cronkright says there doesn't appear to be any human-to-human contact.
Anyone who attended the Washington County Fair and has flu-like symptoms should contact their medical provider or call 1-877-PA-HEALTH.