kos0760,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<select name="" id="">
<option value="0">0</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
<option value="11">11</option>
<option value="12">12</option>
<option value="13">13</option>
<option value="14">14</option>
<option value="15">15</option>
</select>
<select name="" id="">
<option value="0">0</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
<option value="11">11</option>
<option value="12">12</option>
<option value="13">13</option>
<option value="14">14</option>
<option value="15">15</option>
</select>
<select name="" id="" size="4" >
<option value="4">4</option>
<option value="8">8</option>
<option value="16">16</option>
<option value="32">32</option>
</select>
<script>
var s = document.querySelectorAll("select"),
o = s[2].querySelectorAll("option");
function foo() {
for (var b = +s[0].value + +s[1].value, a = s[2].options.length = 0; a < o.length; a++)
+ o[a].value >= b && s[2].appendChild(o[a]);
s[2].options[0].selected = !0
}
s[0].onchange = s[1].onchange = foo;
</script>
</body>
</html>