Iranian-born Danai Muhammadi, 24, allegedly started a fire at Melissa Crook's family home in Chatham, Kent by squirting petrol through the letterbox, The Sun reported.
Muhammadi's wife Melissa, 20, and 15-month-old son Noah died in the arson attack while her 49-year-old father Mark Crook suffered severe burns and died at the hospital.
The couple's marriage had collapsed six months earlier. The man acted out of "spite, anger and resentment", the Maidstone Crown Court heard.
However, Melissa's mother Amanda, 49, and older brother Bohdan, 21, escaped.
The court heard that the petrol had been sprayed at the foot of the stairs -- cutting off any escape route for the sleeping family at the three-bedroom property.