Portsmouth City councillors Lee Hunt, Leo Madden and Jason Fazackarley switched at the same time, giving the Lib Dems a clear majority with 22 seats to the Tories' 18 and Labour's two.
It is the first time one party has had full control of the council since 2000.
Cllrs Fazackerley and Madden - both former Labour members - had been independents while Cllr Hunt was Conservative.
Cllr Hunt said: “Portsmouth South Conservatives have lost their way - they have forgotten that it is people that count, not power.”
Cllr Madden said: “I and Jason have decided to join the Lib Dems because they are a party that puts the interests of the people of Portsmouth first, and will always do so, rather than engaging in political posturing”
According to the local newspaper, the shift won't affect the day-to-day running of the council but the Lib Dems will no longer have to rely on any other party - or the Lord Mayor's casting vote - to get policies through.