int a = 13;
int b = 12;
int c = 8;

void setup() {
pinMode(13, OUTPUT);
pinMode(12, OUTPUT);
pinMode(8, OUTPUT);
}

void loop() {
  digitalWrite(a, HIGH);
  digitalWrite(b, LOW);
  digitalWrite(c, LOW);
  delay(3000);
  digitalWrite(b, LOW);
  delay(3000);
  digitalWrite(c, HIGH);
  delay(3000);
  digitalWrite(c, LOW);
  delay(3000);
  digitalWrite(b, HIGH);
  delay(3000);
  digitalWrite(b, LOW);
  delay(3000);

}