A third victim has died and a man is in police custody following an alleged domestic violence incident at home in Sydney’s south west.

NSW police arrived at the house in Rosemeadow on Sunday at 1.30am after reports of a disturbance.

Officers found a 64-year-old man suffering serious head injuries, along with the bodies of a woman in her 60s and a man in his 20s.

The injured man received medical treatment from NSW Ambulance paramedics and Liverpool Hospital, but he later died.

The bodies are yet to be formally identified.

Police say the victims “were assaulted by a man known to them”.

Police arrested a 32-year-old man and he has been taken to Campbelltown Police Station for questioning.