One GPIO pin for LED, Pushbutton and Buzzer via high-pass filter?
You can of course, instead of directly driving the LED, add a high-impedance low pass filter, driving a thresholding element (e.g. a transistor, a comparator with a reference, or an elegantly connected diode), which in turn drives the LED.

                                     +3.3V
                                       |
                                       R
                                       |
                                      LED
     +----High Pass----Piezo---GND     |
GPIO-+----------------Switch---GND   |/
     +----R_large---+----------------| 
                    |                |\
                    C                  v
                    |                  |
                   GND                GND