prova questo

codice:
...
<head>
<script>
function enableSelect() {
  document.getElementById('myselect').disabled = !document.getElementById('rbutt3').checked;
}
</script>
</head>

<body>

Opt1 <input type="radio" name="groupradio" value="opt1" id="rbutt1" onclick="enableSelect()" />

Opt2 <input type="radio" name="groupradio" value="opt2" id="rbutt2" onclick="enableSelect()" />

Opt3 [abilita select]<input type="radio" name="groupradio" value="opt3" id="rbutt3" onclick="enableSelect()" />



<select id="myselect" disabled="disabled">
  <option>Select</option>
</select>
</body>
...

Ciao