Fabianus Tibo, Marianus Riwu and Dominggus Silva had been set to face the firing squad last month but won a reprieve after a papal appeal.
The men were found guilty of inciting attacks during religious rioting in Central Sulawesi in 2000.
Their supporters and rights groups have questioned the trial's fairness.
Some 4,000 extra troops have been deployed in religiously-divided Sulawesi amid fears the executions could spark further violence.
"If there are unwanted actions, or actions tending toward anarchy, police will not hesitate to take repressive action," Central Sulawesi police chief Badroddin Haiti said.
Lawyers for the three men said provincial prosecutors delivered a letter saying the executions would take place on Thursday 21 September, though one report said it could take place on Friday.
Attorney General Abdul Rachman Saleh told reporters the letter, delivered late on Monday, said the men would be shot in 72 hours time.
The men say they are innocent of the charges that they masterminded a series of attacks on the Muslim community in the Central Sulawesi district of Poso in 2000, killing at least 70 people.
The attack was part of a wave of violence that left more than 1,000 people killed. It was triggered by a brawl between Christian and Muslim gangs in December 1998.
The men's execution was delayed last month after a plea for clemency from Pope Benedict XVI and demonstrations by thousands of Indonesian Christians.
Three Muslim militants are also currently on death row for their part in the 2002 Bali nightclub bombings that killed more than 200 people.