Former president Muhammad Khuna Walad Haid Allah and government opponents Ahmad Walad Daddah and Shaikh Walad Hurma are accused of financing the Cavaliers for Change, an armed insurgency created abroad by renegade military men.
A defence attorney said the three were being held in custody at Wad Naga, a military garrison 50km east of the capital Nouakchott, where 191 people are on trial for allegedly fomenting various coup attempts against the government.
"A villa was specially prepared for them in the interior of the garrison to spend the night before their appearance in criminal court on Tuesday morning," said lawyer Yacuba Diallo.
He said he did not know if the three accused would be released after their court appearance.
The court heard the testimony of five military and civil prisoners on Monday who all pleaded not guilty.
So far, of 155 people who have appeared before the court, only former commander Salih Walad Hananna, the alleged brains behind the attempted overthrows last summer, and commander Abd al-Rahman Walad Mini, self-proclaimed head of the Cavaliers for Change, have pleaded guilty.
"I wanted to change a rotten and illegal regime by way of a coup, similar to that launched on 12 December 1984 by President Walad Taya"
Salih Walad Hananna, alleged brains behind coup attempts
"I wanted to change a rotten and illegal regime by way of a coup, similar to that launched on 12 December 1984 by President Walad Taya," said Hananna, denying charges that he had taken up arms against his country and led a seditious movement.
An aborted June 2003 coup, quelled by loyalist soldiers after a 36-hour gun battle, was followed by two other alleged plots to overthrow the government in August and September.
Critics of President Muawiya Walad Taya's government say the alleged coup plots have provided a pretext for rounding up and banishing the political opposition, in particular Islamist activists who are gaining ground among the country's 2.7 million people.
The pro-Western Taya government itself came to power in a 1984 coup.