I’m going to say three, two murders and a suicide.

Depends on who’s saying it, since it’s not exactly a well defined term. It would be either 3 or 4, unless you get really loose with things.

Generally, a double murder is one person killing two. A murder suicide is one person killing one other, them themselves. So the implication of a double murder suicide is either someone committing a double murder, then killing themselves for three total; or two murder suicides happening in unison.

I guess it depends on the hyphen. Is it a double-murder suicide, or a double murder-suicide