A third arrest has been made over the disappearance and presumed murder of Mount Gambier man Gordon Hamm.

The man, 21, from Mount Gambier man has been charged with being an accessory to murder.

Police allege Mr Hamm, 34, was forced from his car and murdered at Nelson in south-west Victoria in July.

Despite an extensive search, no body has been found.

Two other men from Mount Gambier also have been charged with murder and are in custody in Victoria.