Adding Third Post Box Column: postbox-container-3

I’ve implemented postboxes in CSV 2 POST but I’m not sure how to use a three column layout. Just adding the div doesn’t work because obviously there needs to be a screen option for it. Which I have added to my plugin also. But despite having screen option for a 3rd column layout and the div in place it does not work.

What could I be missing?

<div id="postbox-container-3" class="postbox-container">
                        <?php
                        $this->do_text_boxes( 'normal' );
                        $this->do_meta_boxes( 'normal' );

                        $this->do_text_boxes( 'additional' );
                        $this->do_meta_boxes( 'additional' );

                        // print all submit buttons
                        $this->do_text_boxes( 'submit' );
                        ?>
                    </div>
                    <div id="postbox-container-2" class="postbox-container">
                        <?php
                        $this->do_text_boxes( 'normal' );
                        $this->do_meta_boxes( 'normal' );

                        $this->do_text_boxes( 'additional' );
                        $this->do_meta_boxes( 'additional' );

                        // print all submit buttons
                        $this->do_text_boxes( 'submit' );
                        ?>
                    </div>
                    <div id="postbox-container-1" class="postbox-container">
                    <?php
                        // print all boxes in the sidebar
                        $this->do_text_boxes( 'side' );
                        $this->do_meta_boxes( 'side' );
                    ?>
                    </div>

I have asked the same question on Stack Exchange WordPress if your interested in covering it there also…