void setup(){
  //program setup
  pinMode(21, OUTPUT);
  pinMode(19, OUTPUT);
  pinMode(18, OUTPUT);
}

void loop() {
  //program LED 1
  digitalWrite(21, HIGH);
  delay(500); // this speeds up the simulation
digitalWrite(21, LOW);
delay(500);
//program LED 2
digitalWrite(19, HIGH);
delay(600);
digitalWrite(19, LOW);
//program LED 3
digitalWrite(18, HIGH);
delay(700);
digitalWrite(18, LOW);
delay(700);
}