void setup() {
for (int i=12; i>=3;i--) {
  pinMode(i, OUTPUT);
}
}

void loop() {
  for (int i=12;i>=3;i--) {
  digitalWrite(i, HIGH);
  delay(100);
}
for (int i=3;i<=12;i++) {
  digitalWrite(i, LOW);
  delay(100);
}
}