To my knowledge only one political advocacy group uses that definition. Prior to this group being founded in the last several years, the definition has been four or more killed in a shooting typically unrelated to any other crime. You know incidents like Sandy Hook, Columbine, Virginia Tech, or Las Vegas. You won't really see any legitimate sources use that definition like the Congressional Research service or most academics.
I haven't heard of most of these incidents? Are these from street gangs shooting each other and bystanders or are they what typically dominates the news in the US like Sandy Hook? I feel like if it was the latter these would have had coverage.