The incident occurred as an international Muslim leader flew to Thailand to join a peace-building forum.
The rangers and the housekeeper were travelling in a pickup truck along a road in tambon Sateng Nok when at least five gunmen opened fire on them.
The attackers set fire to the pickup truck and the bodies inside and stole the rangers' M16 assault rifles. Police found more than 100 bullet shells fired from M16 assault rifles and 9mm handguns scattered on the road.
The victims were identified as Somyos Waewpetch, Kampol Mangwong and Panurat Boonthanuwong _ all attached to No41 paramilitary ranger unit _ and Porn Kaewmak, a housekeeper at the ranger camp.
A ranger who rode a motorcycle around 500m ahead of the pickup truck said he found several metal spikes laid on the road.
Before he could signal to the truck, he heard gunfire behind him.
Authorities in Narathiwat yesterday fanned out across a plantation in Cho Airong district and found a large black plastic bag which contained bomb-making materials and ammunition for 9mm handguns.
The materials could be used to make up to 29 bombs.
Authorities yesterday arrested Assan Sama, 23, on suspicion of detonating a bomb which wounded two soldiers early last month.
Yesterday, Abdullah bin Abdul Mohsin al-Turki, secretary-general of the Saudi Arabia-based Muslim World League (MWL), arrived in Bangkok.
He is among several key religious leaders who will attend and international seminar on religion and peace-building co-organised by the MWL, Religions for Peace International and Mahidol University's Institute of Human Rights and Peace Studies, at Siam City Hotel from tomorrow until Wednesday.
The seminar is aimed at fostering understanding and cooperation among people of different faiths, he said upon his arrival at Suvarnabhumi airport.
"I hope that my visit will bring about a fruitful result," Mr Turki said.
Asked about Thailand's southern insurgency, he said it is the responsibility of every country to contribute to or ensure peace.
"This practice is considered right and people have to also give cooperation to restore peace in the country," he said.
"I hope that Thailand would succeed and gain cooperation from neighbouring countries as well as Muslim countries."
Mr Turki said that the protection of the human rights of Muslim minorities around the world is always the top priority of the MWL.
He said Muslim minorities should be assured the same access to equal rights as the majority groups in all countries.