void setup() {
pinMode(7, OUTPUT);
pinMode(6, OUTPUT);
pinMode(5, OUTPUT);
}

void loop() {
for (int step=5; step<=7; step++) {
  digitalWrite(step, HIGH);
  delay(500);
  digitalWrite(step, LOW);
}
for (int step=7; step>=5; step--) {
  digitalWrite(step, HIGH);
  delay(500);
  digitalWrite(step, LOW);
}
}