unsigned long startMillis1;
unsigned long startMillis2;
unsigned long startMillis3;
const unsigned long period1 = 500;
const unsigned long period2 = 1000;
const unsigned long period3 = 330;

int LEDPIN1 = 13;
int LEDPIN2 = 12;
int LEDPIN3 = 8;

void setup()
{
  Serial.begin(115200);  // Start Serial for debugging info
  pinMode(LEDPIN1, OUTPUT);
  pinMode(LEDPIN2, OUTPUT);
  pinMode(LEDPIN3, OUTPUT);
  startMillis1 = millis();  // Initial start time for LED 1
  startMillis2 = millis();  // Initial start time for LED 2
  startMillis3 = millis();  // Initial start time for LED 3
}

void checkTime(unsigned long &Millis, unsigned long period, int LEDPIN)
{
  unsigned long currentMillis = millis();  // Get the current "time"

  if (currentMillis - Millis >= period)
  {
    digitalWrite(LEDPIN, !digitalRead(LEDPIN));  
    Millis = currentMillis;
  }
}

void loop()
{
  checkTime(startMillis1, period1, LEDPIN1);
  checkTime(startMillis2, period2, LEDPIN2);
  checkTime(startMillis3, period3, LEDPIN3);  
}


//https://forum.arduino.cc/t/using-millis-for-timing-a-beginners-guide/483573/2