int arr_pin[3]={17,19,18};
int i,j ;
void setup() {
  // put your setup code here, to run once:
  for(int i = 0 ; i<3; i++){
  pinMode(arr_pin[i], OUTPUT);
  }
  delay(200);
}

void loop() {
  // put your main code here, to run repeatedly:
  for(i = 0; i<3; i++){

  digitalWrite(arr_pin[i], HIGH);
  delay(200);
  digitalWrite(arr_pin[i], LOW);
  delay(200); // this speeds up the simulation
  }

  for(j = 3; j>0; j--){

  digitalWrite(arr_pin[j], HIGH);
  delay(200);
  digitalWrite(arr_pin[j], LOW);
  delay(200); // this speeds up the simulation
  }
}