// an array of the LED pin numbers
// change this to match your LED pins
int led_pins[] = {7, 6, 5};

void setup()
{
  Serial.begin(115200);
  delay(2500);    // ensure Serial is ready
  Serial.println("READY");

  // if you have an array of LED pins you can use
  // it to set all LED pin modes as OUTPUT
  for (int i=0; i < 3; ++i)
  {
    pinMode(led_pins[i], OUTPUT);
  }
}

void loop()
{
  // you would normally do this on the button push
  int result = millis() % 3;
  Serial.print(result);   // DEBUG print

  // turn off any LED that might be on and show
  // the newly selected LED
  for (int i=0; i < 3; ++i)
  {
    digitalWrite(led_pins[i], LOW);
  }
  digitalWrite(led_pins[result], HIGH);
  Serial.print(", turn on LED at pin ");
  Serial.println(led_pins[result]);

  delay(500);
}