The two 18-year-old and one 19-year-old women were held on suspicion of the commission, preparation and instigation of terrorist acts, the Metropolitan Police force said.
"The arrests were made as part of an ongoing intelligence-led operation in connection with an address on Harlesden Road," the police said in a statement, referring to the location of a raid by armed counter-terrorism officers in northwest London on Thursday evening.
Police at the time said that raid had disrupted an active militant plot.
Monday's arrests came days after a man was arrested carrying knives near Prime Minister Theresa May's office in Westminster, and just over a month after a British-born convert to Islam ploughed a car into pedestrians on Westminster Bridge.
On Thursday, armed counter-terrorism officers using tear gas stormed the house in the Willesden area of the capital and shot a woman in her 20s.
She was released from hosital on Sunday, then herself arrested on suspicion of committing terror offences.
Britain has been on its second-highest alert level of "severe" since August 2014, meaning an attack by militants is considered highly likely.
(Reporting by Alistair Smout; Editing by John Stonestreet and Andrew Heavens)
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