int counter = 1;

void setup() {
  Serial.begin(9600);
}

void loop() {
  
  // Use an if statement and modulo to determine if
  // our number is a multiple of 3
  if ( (counter % 3) == 0 ) {
    Serial.println("Fizz");
  } else {
    Serial.println(counter);
  }
  
  // Increment our counter and wait
  counter = counter + 1;
  delay(500);
}

Simulator: https://tinkercad.com/things/1qho5tgaRMD