Just catch the Blur event for the three textboxes and check that the values are valid within each of the three boxes.
E.g.
<input type="text" id="1" value="test" onBlur="javascript:Validate()();">
function Validate(){
if text > 31 {alert (Invalid Date);}.
}
You will need to handle leap year validaation and month validations as well (30, 31 days etc).
So it might be useful to use something like
http://www.the-art-of-web.com/javascript/validate-date/[
^]. You can do this when all the three date elements have been entered.