<script language="javascript" type="text/javascript">
function functn() {
var a = document.getElementById('text1').value;
var b = document.getElementById('text2').value;
if (a == "")
a = 0;
if (b == " ")
b = 0;
var res = parseInt(a) + parseInt(b);
document.getElementById('text3').value = res;
</script>
<input type="text" value="text1">
<input type="text" value="text2" onblur="functn()" >
<<input type="text" value="text3" ></input>