The origin of "drilling" is certainly in the word "drei" (three), but the term actually is "drilling", not "dreiling". In usage other than guns, it means "triplet", one of three siblings born close together. (In German, as in English, the terms "zwilling" (twin) and "drilling" (triplet) are normally plural, but colloquially, people say "He is a twin" or "She is a triplet", meaning that the person is one of close-born siblings.)
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