Army, Navy and Marines. (Or Army, Navy and Air Force, since I guess the Marines are technically under the control of the Navy, I guess.)
What are more things that specifically go together in groups of three? It could be musical groups, historical figures, or just nouns in general like some of the above that are often described in groups of three.
Executive, Legislative, Judicial
Creation, Preservation, Destruction
“Understanding is a three edged sword, your side, their side, and the truth.” ( Babylon 5 )
Blood, pain, death
Good, Evil, Neutral
Drunk, sober, knurd
Alecto, Megaera, and Tisiphone; the Furies.
Larry, Curly, Moe; the Stooges.
Right/Left; Forward/Back; Up/Down; the three directions of movement.
The Three Little Piggies.
Straight, Bisexual, Gay.