The British woman walked out of a house in Lambeth, south London, last month alongside a 57-year-old from Ireland and a 69-year-old Malay following negotiations with a support charity.
Investigating officers said they had “never seen anything of this magnitude before” and described the trio as “deeply traumatised”.
The couple, both 67, were arrested at the property yesterday when details of the investigation into slavery and domestic servitude were released.
Det Insp Kevin Hyland said: “All three women were deeply affected and traumatised. Their lives were greatly controlled. For much of it, they would have been kept on the premises. The women were recovered some weeks ago but the arrests were made this morning.”
It is thought one victim mustered the courage to contact a charity having seen a TV documentary about forced marriages, which resulted in the police becoming involved.
Officers said the 30-year-old woman had allegedly never had contact with the outside world.
Aneeta Prem, founder of the Freedom Charity which worked with police to ensure their release, said the alleged victims are believed to have suffered physical and mental harm, but were able to walk out of the property following the delicate negotiations with its call centre.
She told Sky News: “We started in-depth to talk to them when they could; it had to be pre-arranged. They gave us set times when they were able to speak to us. It was planned that they would be able to walk out of the property. The police were on standby.
“They were able to leave the property. It was a very, very excellent way it happened.”
She said the two people arrested were considered the “heads of the family”, and the women were “absolutely terrified” of them.