Disabled / Enabled multiple selecty

<div class="multipleSelect">
<select name="platforma1" multiple>
<optgroup label="Web design"> 
<option value="win">Windows</option>
<option value="linux">Linux</option>
<option value="unix">Unix</option>
<option value="mac">Mac</option>
</optgroup>
</select>
</div>

<div class="multipleSelect">
<select name="platforma2" multiple>
<optgroup label="Výroba"> 
<option value="win">Windows</option>
<option value="linux">Linux</option>
<option value="unix">Unix</option>
<option value="mac">Mac</option>
</optgroup>
</select>
</div>

<div class="multipleSelect">
<select name="platforma3" multiple>
<optgroup label="Grafika"> 
<option value="win">Windows</option>
<option value="linux">Linux</option>
<option value="unix">Unix</option>
<option value="mac">Mac</option>
</optgroup>
</select>
</div>

var selection = $('#tab3 .multipleSelect');
var selectionChild = $('#tab3 .multipleSelect').children("select");

selectionChild.change(function(){
if($(this).attr("selected",true)){
alert("klik"); // skusal som nieco v zmysle  -> selection.siblings().attr("disabled", true);  // ale znepristupni to vsetky 3 selecty
}
});