const int led1 = 12; 
const int led2 = 11; 
const int led3 = 10; 
  
void setup() { 
  pinMode(led1, OUTPUT); 
  pinMode(led2, OUTPUT); 
  pinMode(led3, OUTPUT); 
} 
  
void loop() { 
  digitalWrite(led1, HIGH);  // only LED 1 ON 
  digitalWrite(led2, LOW); 
  digitalWrite(led3, LOW); 
  delay(400); 
  
  digitalWrite(led1, LOW);   // only LED 2 ON 
  digitalWrite(led2, HIGH); 
  digitalWrite(led3, LOW); 
  delay(400); 
  
  digitalWrite(led1, LOW);   // only LED 3 ON 
  digitalWrite(led2, LOW); 
  digitalWrite(led3, HIGH); 
  delay(400); 
}