html:
<input type="text" class="datepick" id="date_1" />
<input type="text" class="datepick" id="date_2" />
<input type="text" class="datepick" id="date_3" />
script:
$('.datepick').each(function(){
$(this).datepicker();
});
(pseudo coded up a bit to keep it simpler)