djonA,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<script>
function kjhgs() {
var summa = 0;
var go = document.getElementsByClassName('vse');
for (var i = 0; go[i]; i++) {
summa += +go[i].value;
}
document.getElementById("syda").value = summa;
}
</script>
<input value="" id="syda">
<select class="vse" onchange="kjhgs();">
<option value="0"></option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<select class="vse" onchange="kjhgs();">
<option value="0"></option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
</body>
</html>