Most piezos I've seen have just two connections, but this type has three.

What's the third wire for?
They're called self drive types, and they're meant to be used as part of the oscillator:  

  

The piezo effect works both ways: if you apply a voltage the piezo stretches, but also if it stretches it creates a voltage. This principle is used to create a feedback signal which drives the oscillator.
The advantage of the self drive is that it will automagically work at its resonance frequency, where it produces the loudest sound. In 2-wire circuits the oscillator's frequency is independent of the piezo's resonance frequency, and it's the designer who has to make that they're close.   

For the piezo of your picture:  


  "G" = black
  "M" = red
  "F" = blue  


(I guess M, F and G stand for Main, Feedback and Ground, resp. CMIIW)
