Melissa Merritt and her common-law husband Christopher Fattore had already been facing first-degree murder charges in the deaths of Merritt's ex-husband Caleb Harrison and his mother, Bridget Harrison.
Their lawyers say the pair were also charged Thursday with first-degree murder in the death of William Harrison — Caleb Harrison's father and Bridget Harrison's husband.
Caleb Harrison was found dead in August 2013 in a home in Mississauga, Ont. — the same home where his mother died in 2010.
The 63-year-old woman's death was first classified as suspicious, but it was only when officers started investigating her son's death that they concluded both had been asphyxiated.
Police then re-opened an investigation into the 2009 death of William Harrison, who also died in the same home. Investigators had originally determined he died of natural causes.
Lawyers for Merritt and Fattore say their clients plan to fight all the charges against them.
Merritt and Fattore were arrested near Bridgewater, N.S., last year, where they had moved shortly after Caleb Harrison's death.
They have since been transferred to Ontario, where their case is being heard in the city of Brampton.
The case involves an intricate web of family relations.
Merritt and Caleb Harrison had two children and split up in 2005, which is also the year she and Fattore met, police have said.
Merritt was previously convicted of the parental abduction of the children she had with Harrison — an abduction police said took place around the same time as Harrison's father's death.
The two children were living with Harrison at the time of his death, police said.
Fattore and Merritt also had children of their own and all kids are currently being cared for, police have said.