form to post multiple inputs into post_content

I am not sure which form plugin you are using, but one way to achieve this is to use Contact Form 7 form plugin and the extension plugin Post My CF7 Form. It allows you to trigger a filter when mapping your form submission to a custom post, you can then create a custom function hooked on the filter to map all your fields to the post content as you wish to.

The Contact Form 7 allows you to create various fields for your form and lay them out. You then display your form using a shortcode. So assuming you have named your fields as field-1, field-2, field-3, and a title field for your post title. You can map your form using the extension plugin, by mapping your title field to your custom post title and select the filter option from the dropdown options for the post content, this will create a new filter which you can hook using the following function in your functions.php file in your theme folder,

    //assuming that your mapped custom post type is my-cpt
    add_filter('cf7_2_post_filter-my-cpt-editor','filter_my-cpt_content',10,3);
    function filter_my-cpt_content($value, $post_id, $form_data){
      //$value is the post field value to return, by default it is empty
      //$post_id is the ID of the post to which the form values are being mapped to
      // $form_data is the submitted form data as an array of field-name=>value pairs
      $value = '<div id="form-field-content">';
      if(isset($form_data['field-1']){
        $value .= '<p>Field: '. $form_data['field-1'] . '</p>';
      }
      if(isset($form_data['field-2']){
        $value .= '<p>Field: '. $form_data['field-2'] . '</p>';
      }
      if(isset($form_data['field-3']){
        $value .= '<p>Field: '. $form_data['field-3'] . '</p>';
      }
      $value .= '<div/>';
      return $value;
    }

that will populate your content with your 3 fields.