const byte buttonPin = 2; // pin D2 --- button ----  GND
const byte relayPin = 3; // trigger pin of a suitable relay, powered separately

void lampOn() {
  digitalWrite(relayPin, HIGH); // or LOW depending on the state that triggers the relay
}

void lampOff() {
  digitalWrite(relayPin, LOW); // or HIGH depending on the state that release the relay
}

void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(relayPin, OUTPUT);
  while (digitalRead(buttonPin) == LOW); // wait until button is released if needed
}

void loop() {
  if (digitalRead(buttonPin) == LOW) {
    // button was pressed
    delay(3000); // wait for 3s (3000 ms)
    lampOn();    // turn on the flash
    delay(100);  // for 100ms
    lampOff();    // turn off the flash
    while (digitalRead(buttonPin) == LOW); // wait until button is released if needed
    delay(15); // poor's man anti bounce
  } // end if
} // end loop