Flathead County health officials say there are now 39 confirmed cases of pertussis in the valley.
Two of three new cases of the potentially fatal disease are in children who were not immunized.
The health department says the lab will continue to run tests and will have more results on Monday morning.
There are currently nine school systems affected by whooping cough.
There are more than 200 cases across Montana, the highest number since 2005.
Ravalli and Flathead Counties have the most cases. Missoula County is reporting 8 cases so far.