Sian Blake, who had motor neurone disease, played homewrecker Frankie Pierre in the British soap between 1996 and 1997. Her character seduced cafe worker Alan Jackson, who left his wife Carol to be with her. After moving in with Alan, Frankie went on to try and seduce other EastEnders characters before leaving the show.
A police spokesperson said they all died from head and neck injuries.
Ms. Blake's partner, Arthur Simpson-Kent, is being sought by police over the deaths. However, the BBC understands he has left the UK and traveled to Ghana.
Ms. Blake and the couple's sons had not been seen since December 13. They were reported missing three days later.
The case is now being treated as a triple murder inquiry.
Det Supt Paul Monk, from the Met's Homicide and Major Crime Command, said the police were "continuing to appeal for any information about the movements of Arthur Simpson-Kent".
In an interview with ITV, Ava Blake, Ms. Blake's sister, said, "Unfortunately, I believe Arthur was responsible," she said. "I want him to face justice and explain why."
The Met has referred the case to the Independent Police Complaints Commission over how the investigation was handled.
Arthur Simpson-Kent was arrested in Ghana today. He was carrying a knife when officers surrounded him in a coconut plantation - but he did not use it to threaten. Witnesses said ten armed officers scrambled to the beach – and locals pointed out where Simpson-Kent was hiding in the plantation. Simpson-Kent is likely to be flown back to Britain in the next few days.
Sian Blake's aunt Joenie Fearon posted news of Simpson-Kent's arrest on her Facebook page, and a family friend replied: “Great news!”
Another aunt, Fay Golding, who lives in New Jersey, said: “I’m relieved. It won’t compensate much but at least they got hold of him. I’m not happy because obviously I lost my niece and grand-nephews but at least we all feel better that he was found and he’ll have to answer a lot of questions now."