Officers have been undertaking alcohol test purchase operations at licensed premises in Buckshaw Village and Chorley this week.
Of the 13 places visited, three failed and sold alcohol to the police test purchaser - a girl who was only 15-years-old.
"The cashiers in question have all been issued with £90 fixed penalty notices following the sale," a spokesman for Lancashire Police said.
"The premises and person in charge will be revisited by the Police Licensing Team and Chorley Council Licensing team in due course to ensure that they are complying with the conditions of their premises licence."
If you have any concerns over premises selling alcohol to children, report it via the Lancashire Police website HERE.
Similarly, if you have any concerns over premises selling tobacco, e-cigarettes or any other age-restricted items, report it HERE.
What is a test purchase?
Test purchasing is an initiative permitted to police officers under the Licensing Act 2003.
An underage volunteer police cadet is sent into shops and bars to test the premises' resilience around alcohol sales.
The tests are usually undertaken at premises where intelligence indicates that the retailer has either sold age-restricted products to a minor, failed tests in the past or are in a 'hotspot' area.
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