The form name is useless and misleading. Make it an id attribute instead, and your scripting will be able to more easily work with the elements of the form.
Typically this is done by using the name attribute to name the fields of the form that you intend to submit.
<form id="mpr_audit" action="audit_action.php" method="post">
<input type="no1" name="no1" />
<input type="no2" name="no2" />
<input type="no3" name="no3" />
The benefit of doing things this way, is that the only possible conflicting part of the form is now the single id attribute on the form tag, which should be a different identifier on a different form anyway to represent what it is that the form does.
var form = document.getElementById('mpr_audit');
Now that you have a reference to the form, you can easily access the named elements within it by using the elements collection:
var form = document.getElementById('mpr_audit'),
total = 0;
total += Number(form.elements['no1'].value) || 0;
total += Number(form.elements['no2'].value) || 0;
form.elements['no3'].value = total;
The "|| 0" part means that if we don't get a valid number from the field, we will default to using 0 instead.