Hi, I didnt see your flen variable value?
document.getElementById("demo1").innerHTML = text;
This code just will change demo1 innerHtml This is not remove demo1 div element.
You can try document.getElementsByClassName("container3")[0].innerHtml = text;
document.getElementsByClassName("container3")[1].innerHtml = text;
var arr = document.getElementsByClassName("") This code will return array. So, you can try arr[0] for firt item select.