<p>Choose an option in the drop-down list and display that option.</p>
 
<form>
  <select id="mySelect1" onchange="myFunction()">
    <option>+1</option>
    <option>+2</option>
    <option>+3</option>
    <option>+4</option>
  </select>
</form>
<form>
  <select id="mySelect2" onchange="myFunction()">
    <option>+10</option>
    <option>+20</option>
    <option>+30</option>
    <option>+40</option>
  </select>
</form>
<form>
  <select id="mySelect3" onchange="myFunction()">
    <option>+100</option>
    <option>+200</option>
    <option>+300</option>
    <option>+400</option>
  </select>
</form>
 
<p id="demo"></p>
 
<script>
function myFunction() {
  var x = document.getElementById("mySelect1");
  var y = document.getElementById("mySelect2");
  var z = document.getElementById("mySelect3");

  document.getElementById("demo").innerHTML = parseInt(x.value) + parseInt(y.value) + parseInt(z.value)   ;
}
</script>