change month in date to 3-letter abbreviation

The easiest solution is to use three fields and merge them into one with the gform_pre_submission filter. Create your form and add your three fields in the format you want. Use CSS to position them however you like.

http://www.gravityhelp.com/documentation/page/Gform_pre_submission_filter

Add a hidden or admin only field (a single line text) and do not make it required. This is where the merged date field will be stored.

add_action('gform_pre_submission_filter', 'concatenate_date');
function concatenate_date($form) {
  // change input IDs here
  $dd   = $_POST['input_25'];
  $mmm  = $_POST['input_26'];
  $yyyy = $_POST['input_27'];
  // input_37 is the hidden field
  $_POST['input_37'] = $dd . '-' . $mmm . '-' . $yyyy;
  // return modified form object
  return $form;
}

That would take fields 25, 26 and 27 and concatenate them with dashes in between, and store that in field 37.

Let us know if you get stuck. We'll just need to know the field IDs of the three fields you want to string together, and the field ID where you want to store the value.