void setup() 
{
 pinMode (2,OUTPUT);
 pinMode (3,OUTPUT);
 pinMode (4,OUTPUT);
}

void loop() 
{
  int n,i;
  for(n=1;n<=25;n++)
  { 
   digitalWrite(2,1);
   digitalWrite(3,1);
   digitalWrite(4,1);
   delay(1);
   
   for(i=1;i<=19;i++)
   {
   digitalWrite(2,0);
   if(i>=8)
    digitalWrite(3,0);
    if(i>=18)
   digitalWrite(4,0);
   delay(1)
   }
  }
  delay(500);
  
}