const int relayPin = 53;  // Pin connected to the relay (using pin 53 for an Arduino Mega)

void setup() {
  pinMode(relayPin, OUTPUT);
}

void loop() {
  digitalWrite(relayPin, HIGH);  // turn the relay on
  delay(3000);                    // wait for 3 seconds
  digitalWrite(relayPin, LOW);   // turn the relay off
}