// each "event" (LED) gets their own tracking variable
unsigned long previousMillisLED12=0;
unsigned long previousMillisLED13=0;

unsigned long previousMillisLED3=0;
 
// different intervals for each LED
int intervalLED12 = 500;
int intervalLED13 = 5000;

int intervalLED3 = 2000;
 
// each LED gets a state varaible
boolean LED13state = false;     // the LED will turn ON in the first iteration of loop()
boolean LED12state = false;     // need to seed the light to be OFF
 
void setup() {
   pinMode(13, OUTPUT);
   pinMode(12, OUTPUT);

   pinMode(3, OUTPUT);
   
}
void loop() {
   // get current time stamp
   // only need one for both if-statements
   unsigned long currentMillis = millis();
 
   // time to toggle LED on Pin 12?
   if ((unsigned long)(currentMillis - previousMillisLED12) >= intervalLED12) {
      LED12state = !LED12state;
      digitalWrite(12, LED12state);
      // save current time to pin 12's previousMillis
      previousMillisLED12 = currentMillis;
   }
 
// time to toggle LED on Pin 13?
  if ((unsigned long)(currentMillis - previousMillisLED13) >= intervalLED13) {
      LED13state = !LED13state;
      digitalWrite(13, LED13state);
      // save current time to pin 13's previousMillis
      previousMillisLED13 = currentMillis;
  }
}

question:

1. Didn't see the led12 and led13 have 10 times defferency , almost same intervals, why?
2. how can use mill() as simple as delay, is that a must to use if.