Post-mortem examinations are to be carried out today
Gardaí in Kanturk say they are not looking for any other person in connection with the tragedy, but they are appealing for witnesses.
The men - one aged in his 50s and two in their 20s and all members of the same family - are all believed to have sustained gunshot wounds.
They have been named locally as Diarmuid (23) and Mark (25) O'Sullivan and their father, Tadgh (59).
It it believed a dispute may have arisen between the sons over a will and the inheritance of local farmland.
At approximately 6.30am on Monday officers were alerted to an incident at Assolas.
A short time earlier a woman, aged in her 60s, alerted neighbors that guns had been fired at her home.
Gardaí were told that three men were in the house, and that one may have suffered from gunshot wounds.
They responded to the scene and were told that further shots had been heard.
The scene of the incident, a family home, was contained by local uniform and plain clothes Gardaí supported by the Armed Support Unit.
Additional support was also provide by the National Ambulance Service.
Throughout the morning, Garda negotiators attempted to make contact with anyone in the house.
After 1.00pm members of the Emergency Response Unit approached the house and the body of a man in his 20s was discovered with a gunshot wound in a bedroom of the house.
No other people were in the house.