Fire, conflagration, holocaust are comparable when meaning a blaze that reduces or threatens to reduce one or more buildings to ashes.
Fire is the general term referable to such an event, whether it involves one or many buildings and whether it is checked or not.
Conflagration implies a devastating fire that must be contended with by all the available forces; it usually takes a length of time to check it or to prevent its further advance.
Holocaust basically denotes a burnt sacrifice, but in more general use it refers usually to a conflagration in which there has been a great loss of life and especially of human life. In extended use holocaust usually stresses destruction of life, but it may blend in the notion of sacrifice.