void setup() {
  pinMode(3,OUTPUT); // red
  pinMode(9,OUTPUT); // green 
  pinMode(6,OUTPUT); // blue
}

void loop() {
  for (int i=0; i <= 255; i++) {
    analogWrite(3,i);
    analogWrite(6,i);
    analogWrite(9,i);
    delay(20);
    }
    for (int i=255; i >= 0; i--) {
    analogWrite(3,i);
    analogWrite(6,i);
    analogWrite(9,i);
    delay(20);
  
  }
}