// declare an array (list) of pins
// int arrayname[size] = {value1, value2, value3};
int PINS[3] = {11, 12, 13};

void setup() {
  // set the pinMode() using a loop
  for(int i=0 ; i<3 ; i=i+1) {
    pinMode(PINS[i] , OUTPUT);
  }
}

void loop() {
  // loop from the lowest pin to highest
  for(int i=0 ; i<3 ; i=i+1) {
    // turn on the current pin
    digitalWrite(PINS[i] , HIGH);
    // delay for 0.3 seconds
    delay(300);
    // turn off the current pin
    digitalWrite(PINS[i] , LOW);
    // delay for 0.3 seconds
    delay(300);
  }
}