int timer = 2500;
int ledPins[] = {
  8,7,2
};
int pinCount = 3;

void setup() {

  for (int thisPin = 0; thisPin < pinCount; thisPin++)
  pinMode(ledPins[thisPin], OUTPUT);
  }

void loop() {
 
  for(int thisPin = 0; thisPin < pinCount; thisPin++){
    
    digitalWrite(ledPins[thisPin], HIGH);
    delay(timer); 

    
    digitalWrite(ledPins[thisPin], LOW);
  }
  }