void setup() {
  pinMode(13, OUTPUT);
  pinMode(12, OUTPUT);
  pinMode(11, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  for (int red=10; red>=0; red--) {
    digitalWrite(13, HIGH);
    digitalWrite(12, LOW);
    digitalWrite(11, LOW);
    Serial.print("Red time left: ");
    Serial.print(red);
    Serial.println(" seconds");
    delay(1000);
  }

  for (int green=12; green>=0; green--) {
    digitalWrite(13, LOW);
    digitalWrite(12, HIGH);
    digitalWrite(11, LOW);
    Serial.print("Green time left: ");
    Serial.print(green);
    Serial.println(" seconds");
    delay(1000);
  }

  for (int yellow=10; yellow>=0; yellow--) {
    digitalWrite(13, LOW);
    digitalWrite(12, LOW);
    digitalWrite(11, HIGH);
    Serial.print("Yellow time left: ");
    Serial.print(yellow);
    Serial.println(" seconds");
    delay(1000);
  }
}