The former chairman of Public Service Commission, Martin Mahe, registered the constitutional application against President Tallis.
It argues the Constitution has no provisions for the Head of State to pardon convictions, only sentences.
Natuman and Salwai were convicted during their reigns as prime minister in two separate legislatures.
Vohor, on the other hand, was sent to jail when he was Member of Parliament for Santo Rural constituency.
Photo file Vanuatu President Obed Moses Tallis