The attacks happened near Dollis Hill station in Brent between late August and mid-September.
Police believe the perpetrator targeted the Tube station in northwest London to seek out women to follow to a quiet area and then sexually assault.
The Metropolitan Police have urged members of the public to come forward if they saw anything suspicious in the area around the time of the assaults.
The first attack happened on 28 August, when a 30-year-old women was followed by a male at around 11.25pm and sexually assaulted in Fleetwood Road in Brent.
Eight days later on 5 September, a 21-year-old woman was sexually assaulted in nearby Ellesmere Road just after 10pm before a member of the public intervened and the suspect ran away.
Then at 10.15pm on 11 September, another 21-year-old was sexually assaulted in Ellesmere Road, with the suspect fleeing after a member of the public stepped in.
Police are treating the attacks are linked.
Each victim described the man as mixed race, slim build, with facial stubble and wearing black clothing.
Detective Inspector Jonny Newell, who is leading the investigation, said the attacks took place “on a weekend, at a similar time and in very close proximity to both the Underground and each other”.
“It is clear that the perpetrator is waiting at or near the Tube station with the sole purpose of seeking out women to follow to a secluded area before he carries out his attacks,” he said.
“On each occasion the victims have managed to raise the alarm, causing their attacker to flee the scene. We are incredibly grateful to the two members of the public who came to their assistance.
“All of the women have understandably been left shaken and we are doing everything we can to stop this happening again.”
DI Newell urged locals to “think about whether they saw anything suspicious on any of these evenings and to check any CCTV or doorbell footage”.
He said sexual assaults of this nature are “very rare” but extra patrols will be deployed in the area over the next few weeks.
“Tackling violence against women and girls is one of the Met’s top priorities and we are determined to stop this man from causing further harm,” DI Newell added.
Anyone with information has been asked to call 101 and use the reference 8659/11SEP. Alternatively, they can anonymously call Crimestoppers on 0800 555111.
Additional reporting by Press Association