int ledPin = 3; // 3:Red, 5:Blue, 6:Green

void setup() {
    pinMode(ledPin, OUTPUT);
    Serial.begin(9600);
}
void loop() {
   if (Serial.available() > 0) {
       byte data = Serial.read();
       if (data == 'H') {
          digitalWrite(ledPin, HIGH);
          Serial.println("LED: On");
       } else {
          digitalWrite(ledPin, LOW);
          Serial.println("LED: Off");      
       }
   }
}