int led1 = 6, led2 = 7, led3 = 8;


void setup() {
  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
  pinMode(led3, OUTPUT);

}

void loop() {
  
  for(int i=6;i<=8;i++){
  digitalWrite(i, HIGH);
  delay(500);   //ms
  digitalWrite(i, LOW);
}

for(int j=8;j>=6;j--){
  digitalWrite(j, HIGH);
  delay(500);   //ms
  digitalWrite(j, LOW);

}

}