A triple homicide is generally a form of homicide in which three people die during the commission of a single act or event. The key to this meaning is that the three deaths occur either simultaneously or due to a single action or inaction. For example, if a person is robbing a store and while committing the robbery kills three people, that would constitute a triple homicide. On the other hand, if someone kills three people over the course of several days and otherwise unrelated incidents, then that would not be a triple homicide, and would instead simply be three occurrences of homicide.
Homicide is sometimes used synonymously with murder; however, as a legal term it has a larger meaning and is, in fact, inclusive of murder, manslaughter, and justifiable types of homicide. In general, homicide is used to refer to any type of action or inaction by one person, which results in the death of someone. This can be intentional or unintentional, and in the United States (US) it can generally be broken down into one of five different types of homicides. These types of homicide include two different types of murder, first degree criminal homicide and second degree criminal homicide, two forms of manslaughter, which are voluntary and involuntary manslaughter, and justifiable homicide.
A triple homicide would be any single action or inaction in which three individuals die. The two forms of murder both require intent to commit the act, though second degree criminal homicide can occur in the heat of passion but still shows prior intent or homicide committed during another felony crime. Manslaughter typically indicates a lack of intent, and a homicide that is committed either in a moment of anger or one that is accidental but in which a person is still responsible. If, for example, someone accidentally set off an explosive that resulted in the deaths of three people, then it would be a triple homicide and would likely be considered involuntary manslaughter.
In the United Kingdom (UK) similar definitions for types of homicide are applied, and a triple homicide in the UK could be intentional murder or accidental manslaughter. There are also types of justifiable homicide found in many countries, such as self defense and the performance of state sanctioned capital punishment. Someone who is being attacked by three individuals may protect himself or herself and potentially commit triple homicide by killing the attackers, but if a reasonable threat to his or her life was established then it would not necessarily be a criminal act.