int led = 1;

void setup() {
  pinMode(2, OUTPUT);
  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
}

void loop() {
  if(led == 1){
    for(int i = 0; i < 255; i++){
      analogWrite(2, i);
      delay(5);
    }
    for(int i = 255; i > 0; i--){
      analogWrite(2, i);
      delay(10);
    }
    led = 2;
  } else if(led == 2){
    for(int i = 0; i < 255; i++){
      analogWrite(3, i);
      delay(10);
    }
    for(int i = 255; i > 0; i--){
      analogWrite(3, i);
      delay(10);
    }
    led = 3;
  } else if(led == 3){
    for(int i = 0; i < 255; i++){
      analogWrite(4, i);
      delay(10);
    }
    for(int i = 255; i > 0; i--){
      analogWrite(4, i);
      delay(10);
    }
    led = 1;
  }
}