Three inmates have been charged in the death of a San Quentin prison guard who was killed with a makeshift spear last summer, the Marin County district attorney's office said. A spokesman said Andre Johnson, 21, Jarvis J. Masters, 23, and Lawrence Woodard, 39, have been charged with conspiracy and murder with special circumstances in the death of Sgt. Howell Dean Burchfield. Johnson, who was serving a five-year sentence for burglary from San Mateo County, is believed to be the person who actually killed Burchfield, the spokesman said.