function MyFunction1()
var w = parseInt(document.form1.field1.value,10);
var x = parseInt(document.form1.field2.value,10);
var y = parseInt(document.form1.field3.value,10);
var z = w+x+y;
document.form1.field4.value=z
<form name="form1">
Enter W:<input type = text name=field1>
Enter X:<input type = text name=field2>
Enter Y:<input type = text name=field3>
Z = W+X+Y:<input type = text name=field4>
</form>