| Notionally, a very large conflagration.
In popular usage, a three-alarm fire refers to anything more than just a house or car fire. Speculating, this probably is inspired by usage of the phrase in television and film. Technically, the term refers to the number of distinct alarms sent to a particular fire house regarding a single fire. Thus, such a fire receives the attention of three times the ususal firemen (and likely is a large fire, indeed). | 
