const int LED1 = 21;
const int LED2 = 22;
const int LED3 = 23;
void setup() {
pinMode(LED1, OUTPUT);
pinMode(LED2, OUTPUT);
pinMode(LED3, OUTPUT);
}

void loop() {
  digitalWrite(LED1, HIGH);
  delay(500);
  digitalWrite(LED2, HIGH);
  delay(500);
  digitalWrite(LED3, HIGH);
  delay(500);
  digitalWrite(LED1, LOW);
  delay(500);
  digitalWrite(LED2, LOW);
  delay(500);
  digitalWrite(LED3, LOW);
  delay(500);
}