Hello FBerwald,
Just a few double & triple concertos, courtesy of Wikipedia & other sources :
These are all modern works, & excepting the well known works by Mozart,Beethoven & Brahms, Telemann & Vivaldi would be a good start for the earlier works, and there must be others from the 18th &19th centuries.
Barber -Capricorn Concerto for Flute,Oboe &Trumpet
M.Tippett -Triple Concerto Violin,Viola & cello
Casella - " " Violin, Cello & Piano
Mageau - " " Violin, Cello & Piano
Dubrovay - " " Trumpet, Trombone & Tuba
Martino - " " Clarinet, Bass Clarinet & Contrabass Clarinet
B.Wagenaar " " Flute, Cello & Harp
Britten - Double Concerto Violin & Viola
Carter - " " Piano & Harpsichord
Delius - " " Violin & Cello
Henze - " " Oboe & Harp
Ligeti - " " Flute & Oboe
Lutoslawski -" " Oboe & Harp
Martinu - " " Flute & Violin
Quadreny - " " Ondes Martenot & Percussion
Badings - " " Bassoon & Contrabassoon
Marcus.