simple set field value

Hi everyone, i have one simple problem i hope:
in my form i'm using values from fields, to calculate with math some new values. i made hidden fields, to set that results in them, and show after submit, but i cant. here is simple code example:

add_action("gform_post_submission_1", "set_post_content", 10, 2);
	function set_post_content($entry, $form)
	{
              $a= $entry[1];
		$b= $entry[2];
              $c = $a + $b;

              $entry[3] = $c;
        }

this part: $entry[3] = $c;
is not working.
how i can set that new value in field[3] entry?
Thanks