int led[] = {13, 12, 8};  

void setup() {
  pinMode(led[0], OUTPUT);
  pinMode(led[1], OUTPUT);
  pinMode(led[2], OUTPUT);
}

void loop() {
  int i= 0;
  for (i = 0; i < 3; i++) {
    digitalWrite(led[i], HIGH);
    delay(500);
    digitalWrite(led[i], LOW);
  }
}