String command; 

#define LED_PIN 13

void setup() { 
  pinMode(LED_PIN, OUTPUT); 
  Serial.begin(9600);
}

void loop() {
  if(Serial.available() > 0) { 
    command = Serial.readStringUntil('.');
    if(command == "aaa") {
      Serial.println("LED ON");
      digitalWrite(LED_PIN, HIGH);
    } else if(command == "bbb") {
      Serial.println("LED OFF");
      digitalWrite(LED_PIN, LOW);
    } else {
        Serial.println("LED ON/OFF 3");
      for (int i = 0; i < 3; i++) {
        digitalWrite(LED_PIN, HIGH);
        delay(500);
        digitalWrite(LED_PIN, LOW);
        delay(500);
      }
    }
  }
  delay(1000);
}