void setup() {
  // put your setup code here, to run once:
 pinMode(3,OUTPUT);//red
 pinMode(7,OUTPUT); //yellow
 pinMode(12,OUTPUT); //green
}

void loop() {
  digitalWrite(3,HIGH);
  digitalWrite(7,LOW);
  digitalWrite(12,LOW);
  delay(20000);

  digitalWrite(3,LOW);
  digitalWrite(7,LOW);
  digitalWrite(12,HIGH);
  delay(10000);

  digitalWrite(3,LOW);
  digitalWrite(7,HIGH);
  digitalWrite(12,LOW);
  delay(2000);
}