Final answer:
The three individuals that investigated the church fire were Mrs. Irene Salvos, Fire Marshal William L. Beers, and the District Attorney and police.
Step-by-step explanation:
The three individuals that investigated the church fire were Mrs. Irene Salvos, Fire Marshal William L. Beers, and the District Attorney and police. Mrs. Salvos, who escaped the burning building, corroborated the charge made by Chief Croker and others that the doors were locked, cutting off escape by the stairs. Fire Marshal Beers conducted an inquiry into the fire and examined operators and Gremes. The District Attorney and police also listened to Mrs. Salvos' story.