int delay_ms = 200;

int arr_pin[3]={25,26,27};

int i, j;

void setup() {
  for (int i=0; i<3; i++) {
    pinMode(arr_pin[i], OUTPUT);
  }
  delay(200);
}

void loop(){
    // loop from the lowest pin to the highest:
  for (int i = 0; i<3; i++) {
    digitalWrite(arr_pin[i], HIGH);
    delay(300);
    digitalWrite(arr_pin[i], LOW);
    delay(300);
  }


// loop from the highest pin to the lowest:
  for (int j = 3; j>=0; j--) {
    digitalWrite(arr_pin[j], HIGH);
    delay(300);
    digitalWrite(arr_pin[j], LOW);
    delay(300);
  }
}