Australian David Hicks, and Britons Moazzam Begg and Feroz Abbasi, are among the six hand-picked by President George W Bush to go before a military tribunal.
Despite repeated protests of innocence, the three men face charges of being linked to Al Qaeda.
Attorney-General Daryl Williams said Australia would protest if a sentence for Hicks' execution was handed down.
Rallying behind the Attorney-General was the Defence Minister, Robert Hill, who insisted that Hicks should not receive the death sentence.
“I would protest the application of the death penalty, yes, and I presume the Australian government would oppose the death penalty,” Hill said.
The US decision to try six Guantanamo Bay Six in military tribunals has triggered international concern and outrage. In Britain, solicitor Louise Christian has been fighting for justice for Abbasi and other Western detainees.
Though a close US ally, even the United Kingdom led by its Prime Minister Tony Blair has expressed doubts about how fair the trials would be.
Blair told his parliament that “active representations would be made to ensure that the trials take place in accordance with proper international law.”
Human rights groups across the world have protested the military trials and said they violated principles of fair justice.
An appeals court in the US has meanwhile backed its government’s decision to deprive a detained terror-suspect of any legal assistance even though he was born in America.
Held in Guantanamo Bay since December 2001, Yaser Esam Hamdi had his appeal against an earlier ruling denying him the right to consult a lawyer turned down by the Fourth US Circuit Court of Appeals on Wednesday.
A US-born Saudi, Hamdi has been in captivity since his capture in Afghanistan.