const byte pins[] = {5, 4, 3};
const byte pinCount = sizeof pins;
const uint16_t freqs[pinCount] = {440, 494, 554}; // A4 B4 CS5

void setup() {
  for (byte p = 0; p < pinCount; p++ ) pinMode(pins[p], INPUT_PULLUP);
}

void loop() {
  bool isPlaying = false;
  for (byte p = 0; p < pinCount; p++ )
    if (digitalRead(pins[p]) == LOW) {
      tone(A0, freqs[p]);
      isPlaying = true;
      break;
    }
  if (! isPlaying) noTone(A0);
}