[devote]

без цикла:

<div id="id1" style="width: 10px; height: 20px; background: red"></div>
<div id="id2" style="width: 10px; height: 20px; background: red"></div>
<div id="id3" style="width: 10px; height: 20px; background: red"></div>
<script>
 alert(
   document.getElementById('id1').offsetWidth + 
   document.getElementById('id2').offsetWidth + 
   document.getElementById('id3').offsetWidth
 );
</script>