void setup() 
{
  for (int pin = 1; pin <= 6; pin++)
    pinMode(pin, OUTPUT);
}

void loop() 
{
  for (int pin = 1; pin <= 6; pin++)
  {
    if (pin % 3 == 0)
      continue;
    
    digitalWrite(pin, HIGH);
    delay(2000);
    digitalWrite(pin, LOW);
    delay(2000);
  }
  delay(5000);
}