Club Revenge, Lola Lo and the Bulldog were all caught during an operation carried out by Sussex Police on Friday night.
Staff at The Blue Man in Queen’s Road and The View in Hove, were also caught out selling alcohol to volunteers.
The volunteers, who were aged 16 or 17 weren’t carrying false ID.
Chief Superintendent Nev Kemp said: “I am very disappointed that so many venues failed the tests. Selling alcohol to children puts them in danger. They need the protection of adults, not to be used as an easy way to make money.
"We are determined to crack down on those pubs, bars, clubs or restaurants who are willing to serve children. These venues must be aware that they are not only putting children at risk, they are also putting their own livelihoods at risk as we may look for their licences to be revoked."
Eclectic Bars, owners of Lola Lo said: "As a responsible operator, we are naturally most upset that we were one of the five venues in Brighton that, in this instance, failed these important tests. Previous tests have always proved successful. In addition, we employ the services of an outside agency to organise random visits to our venues by under-age people who attempt to purchase alcohol - none of our venues have ever failed these tests.
"We will immediately be undertaking exhaustive re-training of our staff to ensure that the situation does not recur."
Sergeant Ben Hearth, from the Brighton and Hove licensing team added: "We conduct these operations regularly. Each venue will have been reported to police by the public or an incident there involving a child will have been reported to police.
"The licensing laws require the premises management and staff to protect children from harm which means ensuring that children under the age of 18 years cannot purchase alcohol and also, in many cases, preventing them from having access to the venue.
"We will continue to run these operations to make sure that licensees are doing all they can to protect children from harm."
Sussex Police are now considering what action should be taken against the venues.