Answer
Observer A hears nothing. Observer B hears a sonic boom. Observer C has already heard the sonic boom, and now hears the Doppler-shifted sound of the airplane moving away.
Work Step by Step
This is explained on pages 534-535. The plane is outrunning its own sound waves, so observer A has not received any sound waves.
The supersonic object forms a cone of shock waves around the direction of motion. As explained on page 534, this shock wave cone moves along with the source. Observer B hears the sonic boom when the shock wave reaches that location.
Observer C has already heard the sonic boom, and now hears the Doppler-shifted sound of the airplane moving away.