const int MAX_LEDS = 3;
const int LED_PINS[MAX_LEDS] = {5, 4, 3};
const long INTERVALS[MAX_LEDS] = {500, 1000, 2000};

bool ledStates[MAX_LEDS];
unsigned long lastTimes[MAX_LEDS];

void setup() {
  Serial.begin(9600);
  for (int i = 0; i < MAX_LEDS; i++)  {
    pinMode(LED_PINS[i], OUTPUT);
  }
}

void loop() {
  unsigned long currMillis = millis();

  for (int i = 0; i < MAX_LEDS; i++)  {
    if (currMillis - lastTimes[i] >= INTERVALS[i]) {
      lastTimes[i] = currMillis;
      Serial.print("LED: ");
      Serial.println(i + 1);
      ledStates[i] = !ledStates[i];
      digitalWrite(LED_PINS[i], ledStates[i]);
    }
  }
}