The driver, alongside his 20 year old and 35 year old female passengers, died at the accident scene at Middlemount some 5km from Rathdowney, Co Laois.
A 16 year old back seat passenger of the car suffered serious injures and was taken to Tullamore General Hospital.
His condition is now stable and his injuries are not understood to be life-threatening.
The accident occurred shortly after 3am at Middlemount on the Abbeyleix-Rathdowney Road.
It is believed the young man lost control of his car and was unable to prevent it smashing into a heavy metal road sign.
No other vehicle was involved.
He was killed instantly such was the force of the impact with the road sign.
Both female passengers, one in the front passenger seat and the other in the rear of the car, suffered critical injuries and were pronounced dead before they could be transferred to Tullamore General Hospital.
Gardai closed the Rathdowney-Abbeyleix Road and warned that diversions will remain in place until later this afternoon.
Accident scene investigators commenced a detailed examination of the roadway involved in a bid to determine what caused the tragedy.
All three deceased were later transferred to the morgue at Portlaoise General Hospital for post mortem examinations.
The deceased are all from the Rathdowney area and have been friends since childhood.
Two local councillors today admitted it was “a black day not just for Rathdowney but for the entire county of Laois.”
“The entire community is in shock after this – it is an awful tragedy particularly because it involves families that are held in the highest respect and esteem locally,” Councillor John King said.
“It is a black, black day for Rathdowney. There may be major GAA games in Munster and Leinster but there will be no-one celebrating around here,” the Fine Gael official said.
“Stretches of that road are very windy and there was a young man who died not far from there a couple of years back in another accident,”