[Avron2]

<form> 
<input class="s_ch" name="selected_ch[]" value="1"  id="1" type="checkbox" /> 
<input class="s_ch" name="selected_ch[]" value="2"  id="2" type="checkbox" /> 
<input class="s_ch" name="selected_ch[]" value="3"  id="3" type="checkbox" /> 
</form> 
  
<a href="#" id="hidelink" style="display:none">текст</a

$('.s_ch').click(function()  {
if ($('.s_ch:checked').size() > 0)    {
       $('#hidelink').fadeIn(500);
}
else    {
       $('#hidelink').fadeOut(500);
}
});