The victims have been identified as a Malaysian woman, 69, an Irish woman, 57, and a British woman, 30.
Police arrested two people in their 60s at their home in south London.
The women, who are said to be "highly traumatised", are now in safe accommodation, the police said.
The Metropolitan Police said Freedom charity, which aims to advise and support victims of forced marriages or honour- based violence, got in touch last month after it received a call from a woman saying she had been held against her will in a house in London for over 30 years.
"We applaud the actions of Freedom Charity and are working in partnership to support these victims who appear to have been held for over 30 years," Detective Inspector Kevin Hyland, from Scotland Yard's Human Trafficking, said.
"We have launched an extensive investigation to establish the facts surrounding these very serious allegations."
The two people arrested were considered the "heads of the family", and the women were said to be "absolutely terrified" of them.
Aneeta Prem, Freedom Charity founder, said the alleged victims have suffered physical and mental harm and were able to walk out of the property after extensive calls with the charity.
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