A third person, said to be a member of area militia groups, plead guilty, on Friday, to a federal weapons charge, at the U-S District Court, in Pittsburgh.     Rimersburg area resident Marvin Hall entered the plea and will be formally sentenced on June 25th.     Hall is a convicted felon who is not allowed to own a gun.     His girlfriend, Melissa Huet, was charged with aiding a convicted felon in possessing a fiream.     She has been granted an extension of time to file pretrial motions.    As a  part of his plea agreement, Hall wll forfeit a shotgun and rifle.     Hall could now be sentenced to receive up to 15 years in prison.     Back in 1999, Hall served over two years in jail for having an unregistered weapon.     as previously reported, Morgie Jones of the Lucinda area and Perry Landis of Dubois plea guiltyo illegal weapons sales but both men were sentenced to the time they had already served.