A last-minute legal challenge by the relatives of Imam Samudra, Amrozi Nurhasyim and Ali Ghufron "will not change or delay the execution," said Djoko Sarwoko.
The men were convicted in the October 2002 Bali nightclub bombings that killed 202 people and authorities have said they could be shot by a firing squad this month.
They were moved to isolation cells on Saturday and execution spots have been prepared on the Nusakambangan island prison where they are being held, said local chief prosecutor Muhammad Yamin. They will be "executed simultaneously" but at different locations, he said.
Those measures and the arrival of two ambulances were seen as indications that prosecutors could carry out the death sentences at any moment.
The 2002 Bali attack was blamed on the Southeast Asian terror network Jemaah Islamiyah and Indonesian police have ramped up security in case of retaliatory attacks.
Jemaah Islamiyah is said to have been behind at least three other suicide bombings in Indonesia since 2002, including a second Bali bombing in 2005 when 21 people died in multiple restaurant blasts.
