Three is a common number in ensembles.
The most common pattern for this is for two members of the group to be each other's Foils, frequently coming into conflict with each other, with the third member of the group acting as the mediator, keeping The Team together and balancing their respective personalities (in which case, see Freudian Trio). This is the dynamic that makes the ensemble powerful. In fact on almost any metric of personality, appearance, or skill, you could break the trio down to two extremes and a medium.
Note the word 'ensemble'. These characters interact directly with each other. You can have three characters in a story that serve as foils of one another without it being a Power Trio.
Power Trios can be embedded in larger ensembles. Three Plus Two is a Power Trio within a Five-Man Band. The Five Temperament version of the Four-Temperament Ensemble sometimes manifests as two Power Trios with the same central member. A Six Student Clique is often broken into two Power Trios and Magnificent Seven Samurai often have one, and sometimes two, Power Trios of their own.
Below, we list the many variations of a Power Trio. Freudian Trio is one of the more frequently found versions and is often the basis of the other trios. They can overlap.
- Balance, Speed, Strength Trio: One balanced, one weak-but-fast and one strong-but-slow.
- Beauty, Brains and Brawn: The Chick, The Smart Gal, and The Big Gal.
- Big, Thin, Short Trio: One big, one thin, and one short.
- Blonde, Brunette, Redhead: One with blonde hair, one with brown/black hair, and one with red hair.
- Chromatic Arrangement: Three characters Colour-Coded for Your Convenience, most commonly as red, blue, and either green or yellow.
- Combat, Diplomacy, Stealth: One fights, one negotiates, and one completes objectives without ever being spotted.
- Comic Trio: An idiot that comes up with stupid plans, another idiot who blindly follows instructions, and a relatively normal person who tries and fails to stop them.
- Fire/Ice/Lightning: A Power Trio of elemental powers. Usually analogous are The Spock to Ice, The Kirk to Lightning, and The McCoy to Fire.
- Fighter, Mage, Thief: A Mighty Glacier, a Squishy Wizard, and a Fragile Speedster.
- Freudian Trio: The three characters represent the Ego, Superego, and Id of Freudian psychology.
- Girl Posse: An Alpha Bitch, a Beta Bitch, and a Brainless Beauty.
- The Hecate Sisters: Three female characters representing the maiden, the mother, and the crone.
- Knight, Knave and Squire: One idealistic fighter, one pragmatic fighter, and one inexperienced fighter.
- Land, Sea, Sky: One of land, one of sea, and one of sky.
- Nice Mean And In Between: A nice character, a mean character, and a character who sticks in-between the two.
- Rock Trio: Three-man rock band containing guitar, bass, and drums.
- Terrible Trio: A villain with two henchmen.
- ¡Three Amigos!: The Hero, a friend of the same gender, and a friend of the opposite gender.
- The Three Faces of Adam: Three male characters roughly representing adventure, authority, and wisdom.
- The Three Faces of Eve: Three female characters representing the child, the wife, and the seductress.
- Three Successful Generations: The youth who will make something of himself, the adult who is making something of himself, and elder who did make something of himself.
- Token Trio: A male of the majority, a Token Minority male, and a Token Female (just as likely to be of the majority or another minority). Frequently overlaps with ¡Three Amigos!.
- Town Girls: Where one is butch, one is femme, and one is neither.
- Two Girls and a Guy: The Heroine and her best friend, and a guy who is just a friend.
- Two Guys and a Girl: The Hero vs. The Lancer, mediated by (and sometimes fighting over) The Chick.
- The Weird Sisters: A trio of supernatural women, such as witches, fairy godmothers, or female seers.
- With a Friend and a Stranger: Central character, joined by a childhood friend and another character from out of nowhere.