var counter = 0;
    
function handleEvent()
{
    var x =event.keyCode;
    //Tab
    if(x==9)
    {
        return;
    }
    //Delete
    if(x==8)
    {
        if(counter!=0)
        counter--;
    }
    else
    {
        if(event.shiftKey)
        {
            event.returnValue=false;
        }
        else  if(event.shiftKey)
        {
            event.returnValue=false;
        }
        else  if(event.shiftKey)
        {
            event.returnValue=false;
        }
        else
        {
           if (!(x >= 48 && x <= 57 || x >= 96 && x <= 105))
           {
               event.returnValue=false;
           }
           else
           {
               if(counter>=3)
               {
                  event.returnValue=false;
               }
               else
               {
                   counter ++;
               }

           }
        }
    }

}