#define LED1 18
#define LED2 19
#define LED3 21


void setup() {
  pinMode(LED1, OUTPUT);
  pinMode(LED2, OUTPUT);
  pinMode(LED3, OUTPUT);
}

void loop() {
  for (int i = LED1; i <= LED3; i++) {
    digitalWrite(i, HIGH);
    delay(200);
    digitalWrite(i, LOW);
  }

  for (int i = LED3; i >= LED1; i--) {
    digitalWrite(i, HIGH);
    delay(200);
    digitalWrite(i, LOW);
  }
}