Ten new episodes of the show, which stars US actor Jeremy Piven as the title character, will be filmed this year.
The drama, inspired by the life of entrepreneur Harry Gordon Selfridge whose business still stands in Oxford Street, has proved a hit with viewers, pulling in audiences of 6.8 million on Sunday nights.
A replica of the shop floor has been recreated in a former carpet warehouse in London, with the Chatham dockyard in Kent standing in for the shop's exterior.
ITV Studios executive producer Kate Lewis said: "We have some surprises in store for an audience in this third series. You only have to walk down Oxford Street today to know that Selfridges department store continued to be successful, but for its founder, Harry Selfridge, things were very different.
"His story was a rollercoaster ride that ended rather tragically. We pick up series three in 1919, the point at which his life really begins to unravel."