Final answer:
The trumpet, trombone, and cornet are brass instruments that change pitch by altering the length of the air column inside the tube, using a sliding tube or valves. They produce both fundamental frequencies and overtones, creating unique timbres for each instrument.
Step-by-step explanation:
The trumpet, trombone, and cornet all belong to the brass family of musical instruments and share some common characteristics. All these instruments can be modeled as modified tubes with symmetrical or anti-symmetrical boundary conditions. They have mechanisms to alter the length of the resonating air column within the instrument, thereby changing the frequency of the note played. In the case of the trombone, this is achieved through a sliding tube, whereas the trumpet and cornet use valves to change pitch. Each brass instrument produces a range of sounds including the fundamental frequency and various overtones, which combine to give each instrument a unique timbre.