int led1=2;
int led2=3;
int led3=4;
int rec;
void setup() {

Serial.begin(9600);
  Serial.println("Hello Arduino\n");
pinMode(led1,OUTPUT);
pinMode(led2,OUTPUT);
pinMode(led3,OUTPUT);
}

void loop() {

if(Serial.available()>0)
{
rec=Serial.read();
if(rec== 'r')
{
digitalWrite(led1,HIGH);
delay(500);
digitalWrite(led1,LOW);
}
if(rec== 'g')
{
digitalWrite(led2,HIGH);
delay(500);
digitalWrite(led2,LOW);
}
if(rec== 'y')
{
digitalWrite(led3,HIGH);
delay(500);
digitalWrite(led3,LOW);
}
}

}