Police and council licensing officers visited 12 licensed premises in the Witney area and two young test purchasers were served at three of them. One sale was in a supermarket and the other two in pubs.
In each case the member of staff involved was issued with a £90 fixed penalty notice.
Councillor David Harvey, council cabinet member with responsibility for licensing said: “It is disappointing that three premises sold alcohol to under-18s.
“Fortunately the majority of licensees in the district are responsible and work with the police and licensing officers to tackle crime and disorder.”
West Oxfordshire District Council licensing officers will work with the three offenders to improve staff awareness and ensure that appropriate procedures are followed to prevent unlawful sales.