const int LED_PINS[] = {12, 11, 10};
const unsigned long INTERVAL1 = 500;
const unsigned long INTERVAL2 = 1000;
const unsigned long INTERVAL3 = 1500;

int ledValues[] = {0, 0, 0};
unsigned long prev1Time = 0;
unsigned long prev2Time = 0;
unsigned long prev3Time = 0;

void setup() {
  for (int i = 0; i < 3; i++) {
    pinMode(LED_PINS[i], OUTPUT);
  }
}

void loop() {
  if (millis() - prev1Time >= INTERVAL1)  {
    prev1Time = millis();
    // do whatever here when INTERVAL1 expires
    ledValues[0] = !ledValues[0];
  }
  if (millis() - prev2Time >= INTERVAL2)  {
    prev2Time = millis();
    // do whatever here when INTERVAL2 expires
    ledValues[1] = !ledValues[1];
  }
  if (millis() - prev3Time >= INTERVAL3)  {
    prev3Time = millis();
    // do whatever here when INTERVAL3 expires
    ledValues[2] = !ledValues[2];
  }
  digitalWrite(LED_PINS[0], ledValues[0]);
  digitalWrite(LED_PINS[1], ledValues[1]);
  digitalWrite(LED_PINS[2], ledValues[2]);
}