The passage where the attacks took place was created by work on the Crossrail project and had become a popular shortcut from Sutton Row to Charing Cross Road.
The first assault was on a 25-year-old woman last November in the alley off Falconberg Mews.
Then on April 14 a 19-year-old office worker was attacked by two men after being dragged off the street. She had become separated from friends in the early hours.
In nearby Covent Garden on April 2 a 25-year-old German tourist was also raped after being dragged into an alley off Maiden Lane. She was grabbed as she smoked a cigarette outside an Italian restaurant.
A Westminster council spokeswoman said businesses had been given safety advice and more than 100 free alarms had been handed out to women in Soho. She added that further work to block off Falconberg Mews was also going ahead and extra lighting had been installed on the building site.
Dean Ingledew, Westminster’s operational director for street management, said: “We would urge everyone, but in particular women, to decide on how they are going to get home and the route they will take before they start a night out. We will continue to work with TfL and the police to develop safe routes home through the night and have moved swiftly to make this particular part of Soho safer.”
He said it would be sensible if women carried attack alarms in the area.
Mr Ingledew added: “The chances of becoming a victim of crime are very small but we want people to be as safe as possible. Attack alarms cost less than a glass of wine and are easily available online, carrying one, even if it is just for peace of mind, makes perfect sense.”
Scotland Yard said there had been no arrests and appealed for information about the attacks and witnesses to contact police.