Music:
* Classical:
* The Three Tenors: Luciano Pavarotti, Placido Domingo, José Carreras
* The Piano Trio: (many famous, but some notable ones include)
* The Beaux Arts Trio: (violinist Isaac Stern, violist Daniel Guilet, cellist Yo-Yo Ma)
* The Emerson String Quartet: (violinists Eugene Drucker and Philip Setzer, violist Lawrence Dutton, cellist Paul Watkins)
* Jazz:
* Oscar Peterson Trio: Oscar Peterson (piano), Ray Brown (bass), Herb Ellis (guitar)
* The Dave Brubeck Quartet: Dave Brubeck (piano), Paul Desmond (alto saxophone), Eugene Wright (bass), Joe Morello (drums)
* The Bad Plus: Ethan Iverson (piano), Reid Anderson (bass), Dave King (drums)
* Rock/Pop:
* Fleetwood Mac: Mick Fleetwood (drums), John McVie (bass), Lindsey Buckingham (guitar, vocals)
* Crosby, Stills & Nash: David Crosby, Stephen Stills, Graham Nash
* The Police: Sting (bass, vocals), Andy Summers (guitar), Stewart Copeland (drums)
* Country:
* The Highwaymen: Johnny Cash, Waylon Jennings, Willie Nelson, Kris Kristofferson
* The Judds: Naomi Judd, Wynonna Judd
* Hip-hop:
* Run-DMC: Run, DMC, Jam Master Jay
* Outkast: Andre 3000, Big Boi
* Migos: Quavo, Offset, Takeoff
Film & Television:
* Charlie's Angels: Farrah Fawcett, Kate Jackson, Jaclyn Smith
* The Three Stooges: Moe Howard, Larry Fine, Curly Howard (later replaced by Shemp Howard)
* The Golden Girls: Bea Arthur, Betty White, Rue McClanahan, Estelle Getty
* The Simpsons: Homer, Marge, Bart
Literature:
* The Three Musketeers: Athos, Porthos, Aramis
Mythology & Folklore:
* The Three Fates: Clotho, Lachesis, Atropos
* The Three Witches: (from Shakespeare's "Macbeth")
* The Three Little Pigs: (in the classic children's story)
This is by no means an exhaustive list, but it gives you a good starting point for exploring famous trios in various fields.