void setup() {
  Serial.begin(9600);  
  pinMode(11, OUTPUT);  
  pinMode(12, OUTPUT);  
  pinMode(13, OUTPUT);  
}

void loop() {
  digitalWrite(11, LOW);
  digitalWrite(12, LOW);
  digitalWrite(13, LOW);
  
  delay(100);
  
  digitalWrite(11, HIGH);
  digitalWrite(12, LOW);
  digitalWrite(13, LOW);
  
  delay(100);
  
  digitalWrite(11, LOW);
  digitalWrite(12, HIGH);
  digitalWrite(13, LOW);
  
  delay(100);
}