Your circuit should look something like this:

And here is code that will flash an LED whenever a variable is a multiple of 3:

const int led_pin = 5;
int counter = 1;
void setup() {
  Serial.begin(9600);
  pinMode(led_pin, OUTPUT);
}
void loop() {
  
  // Use an if statement and modulo to determine if
  // our number is a multiple of 3
  if ( (counter % 3) == 0 ) {
    digitalWrite(led_pin, HIGH);
    delay(500);
    digitalWrite(led_pin, LOW);
  } else {
    Serial.println(counter);
  }
  
  // Increment our counter and wait
  counter = counter + 1;
  delay(500);
}