In a 3-player game, give the starting player the shared 4th Prisoner Marker.
Each player controls their own Prisoner with the 4th Prisoner being a dummy player. A full set of Prisoner materials is set at the table as though a live player was seated. Control of the 4th Prisoner is shared, beginning with the host or owner of the game, who takes the 4th Prisoner marker. When the 4th Prisoner is scheduled to take their turn, the holder of the marker does so. After which, the marker is passed clockwise to the next player for the following round.