For every wind instrument listed except trombone, start by buying the Kendor Recital Solos book. Every solo in each book can be played as a class 3.

Since the Kendor book isn't on the trombone list, buy the Rubank Trombone/Baritone Solos book. There are 4 class 3 solos there, and some good class 2's as they mature. Other good t-bone solos:

Trombone Trip - Eveland
Ruby from Trombone Gems - Vandercook
Rigaudon - Rameau/Dishinger
Saraband - Handel/Barr
Medici Masterworks, Vol. 1 - arr. Dishinger