What is the 3 click sound when the autopilot is disconnected on Airbus aircraft?
There are two separate aural warnings that you might be referring to: one is indeed called 'Triple Click' and consists of three brief clicks, but is emitted when the landing capability becomes degraded or the autopilot otherwise changes mode.

The warning generated upon autopilot disconnection, instead, is the so-called 'Cavalry Charge' and consists of three longer beeping sounds; it's either played once or repeated continuously (cases 1 and 2 below).

From a table in the Airbus A320 FCOM, DSC-31-10 P 4/6:


  TRIPLE CLICK
  
  Meaning: Landing capability downgrade or some cases of mode reversion.
  
  Duration: 0.5 s (3 pulses)
  
  CAVALRY CHARGE 
  
  Meaning:
  
  
  A/P disconnection by takeover push-button.
  A/P disconnection due to failure.
  
  
  Duration:
  
  
  1.5 s
  Permanent
  
  
  Cancellation:
  
  
  Second push on takeover push-button.
  Press Master Warning light or takeover push-button.
  


See this YouTube video (at 2:52) for a cavalry charge immediately followed by a triple click (thank you Jack).