Three workers were killed and 12 others received burn injuries in an explosion in a factory in Mahabubnagar district of Andhra Pradesh today.The incident occurred when the workers were engaged in melting iron scraps in a boiler of the factory near Shadnagar town, police said.There were about 20 workers present at the time of incident in the factory.The injured, who mostly hail from Uttar Pradesh and Bihar, were shifted to Osmania government hospital.